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{{mergefrom|Direct sum|date=January 2014}} | |||
{{for|the broader use of the term in mathematics|Direct sum}} | |||
In [[abstract algebra]], the '''direct sum''' is a construction which combines several [[module (mathematics)|modules]] into a new, larger module. The direct sum of modules is the smallest module which contains the given modules as submodules with no "unnecessary" constraints, making it an example of a [[coproduct]]. Contrast with the [[direct product]], which is the [[duality (category theory)|dual]] notion. | |||
The most familiar examples of this construction occur when considering [[vector space]]s (modules over a [[field (mathematics)|field]]) and [[abelian group]]s (modules over the ring '''Z''' of [[integer]]s). The construction may also be extended to cover [[Banach space]]s and [[Hilbert space]]s. | |||
== Construction for vector spaces and abelian groups == | |||
We give the construction first in these two cases, under the assumption that we have only two objects. Then we generalise to an arbitrary family of arbitrary modules. The key elements of the general construction are more clearly identified by considering these two cases in depth. | |||
=== Construction for two vector spaces === | |||
Suppose ''V'' and ''W'' are [[vector space]]s over the [[field (mathematics)|field]] ''K''. The [[cartesian product]] ''V'' × ''W'' can be given the structure of a vector space over ''K'' {{harv|Halmos|1974|loc=§18}} by defining the operations componentwise: | |||
* (''v''<sub>1</sub>, ''w''<sub>1</sub>) + (''v''<sub>2</sub>, ''w''<sub>2</sub>) = (''v''<sub>1</sub> + ''v''<sub>2</sub>, ''w''<sub>1</sub> + ''w''<sub>2</sub>) | |||
* α (''v'', ''w'') = (α ''v'', α ''w'') | |||
for ''v'', ''v''<sub>1</sub>, ''v''<sub>2</sub> ∈ ''V'', ''w'', ''w''<sub>1</sub>, ''w''<sub>2</sub> ∈ ''W'', and α ∈ ''K''. | |||
The resulting vector space is called the ''direct sum'' of ''V'' and ''W'' and is usually denoted by a plus symbol inside a circle: | |||
:<math>V \oplus W</math> | |||
It is customary to write the elements of an ordered sum not as ordered pairs (''v'', ''w''), but as a sum ''v'' + ''w''. | |||
The subspace ''V'' × {0} of ''V'' ⊕ ''W'' is isomorphic to ''V'' and is often identified with ''V''; similarly for {0} × ''W'' and ''W''. (See ''internal direct sum'' below.) With this identification, every element of ''V'' ⊕ ''W'' can be written in one and only one way as the sum of an element of ''V'' and an element of ''W''. The [[dimension of a vector space|dimension]] of ''V'' ⊕ ''W'' is equal to the sum of the dimensions of ''V'' and ''W''. | |||
This construction readily generalises to any [[finite set|finite]] number of vector spaces. | |||
=== Construction for two abelian groups === | |||
For [[abelian group]]s ''G'' and ''H'' which are written additively, the [[direct product]] of ''G'' and ''H'' is also called a direct sum {{harv|Mac Lane|Birkhoff|1999|loc=§V.6}}. Thus the [[cartesian product]] ''G'' × ''H'' is equipped with the structure of an abelian group by defining the operations componentwise: | |||
* (''g''<sub>1</sub>, ''h''<sub>1</sub>) + (''g''<sub>2</sub>, ''h''<sub>2</sub>) = (''g''<sub>1</sub> + ''g''<sub>2</sub>, ''h''<sub>1</sub> + ''h''<sub>2</sub>) | |||
for ''g''<sub>1</sub>, ''g''<sub>2</sub> in ''G'', and ''h''<sub>1</sub>, ''h''<sub>2</sub> in ''H''. | |||
Integral multiples are similarly defined componentwise by | |||
* ''n''(''g'', ''h'') = (''ng'', ''nh'') | |||
for ''g'' in ''G'', ''h'' in ''H'', and ''n'' an [[integer]]. This parallels the extension of the scalar product of vector spaces to the direct sum above. | |||
The resulting abelian group is called the ''direct sum'' of ''G'' and ''H'' and is usually denoted by a plus symbol inside a circle: | |||
:<math>G \oplus H</math> | |||
It is customary to write the elements of an ordered sum not as ordered pairs (''g'', ''h''), but as a sum ''g'' + ''h''. | |||
The subgroup ''G'' × {0} of ''G'' ⊕ ''H'' is isomorphic to ''G'' and is often identified with ''G''; similarly for {0} × ''H'' and ''H''. (See ''internal direct sum'' below.) With this identification, it is true that every element of ''G'' ⊕ ''H'' can be written in one and only one way as the sum of an element of ''G'' and an element of ''H''. The [[rank of an abelian group|rank]] of ''G'' ⊕ ''H'' is equal to the sum of the ranks of ''G'' and ''H''. | |||
This construction readily generalises to any [[finite set|finite]] number of abelian groups. | |||
== Construction for an arbitrary family of modules == | |||
One should notice a clear similarity between the definitions of the direct sum of two vector spaces and of two abelian groups. In fact, each is a special case of the construction of the direct sum of two [[module (mathematics)|modules]]. Additionally, by modifying the definition one can accommodate the direct sum of an infinite family of modules. The precise definition is as follows {{harv|Bourbaki|1989|loc=§II.1.6}}. | |||
Let ''R'' be a ring, and {''M''<sub>''i''</sub> : ''i'' ∈ ''I''} a [[indexed family|family]] of left ''R''-modules indexed by the [[Set (mathematics)|set]] ''I''. The ''direct sum'' of {''M''<sub>''i''</sub>} is then defined to be the set of all sequences <math>(\alpha_i)</math> where <math>\alpha_i \in M_i</math> and <math>\alpha_i = 0</math> for [[cofinitely many]] indices ''i''. (The [[direct product]] is analogous but the indices do not need to cofinitely vanish.) | |||
It can also be defined as [[function (mathematics)|functions]] α from ''I'' to the [[disjoint union]] of the modules ''M''<sub>''i''</sub> such that α(''i'') ∈ ''M''<sub>''i''</sub> for all ''i'' ∈ ''I'' and α(''i'') = 0 for [[cofinitely many]] indices ''i''. These functions can equivalently be regarded as [[compact support|finitely supported]] sections of the [[fiber bundle]] over the index set ''I'', with the fiber over <math>i \in I</math> being <math>M_i</math>. | |||
This set inherits the module structure via component-wise addition and scalar multiplication. Explicitly, two such sequences (or functions) α and β can be added by writing <math>(\alpha + \beta)_i = \alpha_i + \beta_i</math> for all ''i'' (note that this is again zero for all but finitely many indices), and such a function can be multiplied with an element ''r'' from ''R'' by defining <math>r(\alpha)_i = (r\alpha)_i</math> for all ''i''. In this way, the direct sum becomes a left ''R''-module, and it is denoted | |||
:<math> \bigoplus_{i \in I} M_i. </math> | |||
It is customary to write the sequence <math>(\alpha_i)</math> as a sum <math> \Sigma \alpha_i</math>. Sometimes a primed summation <math> \Sigma ' \alpha_i</math> is used to indicate that [[cofinitely many]] of the terms are zero. | |||
== Properties == | |||
* The direct sum is a [[submodule]] of the [[direct product]] of the modules ''M''<sub>''i''</sub> {{harv|Bourbaki|1989|loc=§II.1.7}}. The direct product is the set of all functions ''α'' from ''I'' to the disjoint union of the modules ''M''<sub>''i''</sub> with ''α''(''i'')∈''M''<sub>''i''</sub>, but not necessarily vanishing for all but finitely many ''i''. If the index set ''I'' is finite, then the direct sum and the direct product are equal. | |||
* Each of the modules ''M''<sub>''i''</sub> may be identified with the submodule of the direct sum consisting of those functions which vanish on all indices different from ''i''. With these identifications, every element ''x'' of the direct sum can be written in one and only one way as a sum of finitely many elements from the modules ''M''<sub>''i''</sub>. | |||
* If the ''M''<sub>''i''</sub> are actually vector spaces, then the dimension of the direct sum is equal to the sum of the dimensions of the ''M''<sub>''i''</sub>. The same is true for the [[rank of an abelian group|rank of abelian groups]] and the [[length of a module|length of modules]]. | |||
* Every vector space over the field ''K'' is isomorphic to a direct sum of sufficiently many copies of ''K'', so in a sense only these direct sums have to be considered. This is not true for modules over arbitrary rings. | |||
* The [[tensor product]] distributes over direct sums in the following sense: if ''N'' is some right ''R''-module, then the direct sum of the tensor products of ''N'' with ''M''<sub>''i''</sub> (which are abelian groups) is naturally isomorphic to the tensor product of ''N'' with the direct sum of the ''M''<sub>''i''</sub>. | |||
* Direct sums are also commutative and associative (up to isomorphism), meaning that it doesn't matter in which order one forms the direct sum. | |||
* The group of ''R''-linear homomorphisms from the direct sum to some left ''R''-module ''L'' is naturally isomorphic to the [[direct product]] of the groups of ''R''-linear homomorphisms from ''M''<sub>''i''</sub> to ''L'': | |||
*::<math>\operatorname{Hom}_R\biggl( \bigoplus_{i \in I} M_i,L\biggr) \cong \prod_{i \in I}\operatorname{Hom}_R\left(M_i,L\right).</math> | |||
*:Indeed, there is clearly a homomorphism ''τ'' from the left hand side to the right hand side, where ''τ''(''θ'')(i) is the ''R''-linear homomorphism sending ''x''∈''M''<sub>''i''</sub> to ''θ''(''x'') (using the natural inclusion of ''M''<sub>''i''</sub> into the direct sum). The inverse of the homomorphism ''τ'' is defined by | |||
*:<math> \tau^{-1}(\beta)(\alpha) = \sum_{i\in I} \beta(i)(\alpha(i))</math> | |||
*:for any ''α'' in the direct sum of the modules ''M''<sub>''i''</sub>. The key point is that the definition of ''τ''<sup>−1</sup> makes sense because ''α''(''i'') is zero for all but finitely many ''i'', and so the sum is finite. | |||
*:In particular, the [[dual space|dual vector space]] of a direct sum of vector spaces is isomorphic to the [[direct product]] of the duals of those spaces. | |||
*The ''finite'' direct sum of modules is a [[biproduct]]: If | |||
*::<math>p_k: A_1 \oplus \cdots \oplus A_n \to A_k</math> | |||
*:are the canonical projection mappings and | |||
*::<math>i_k: A_k \mapsto A_1 \oplus \cdots \oplus A_n </math> | |||
*:are the inclusion mappings, then | |||
*::<math>i_1 \circ p_1 + \cdots + i_n \circ p_n</math> | |||
*:equals the identity morphism of ''A''<sub>1</sub> ⊕ ··· ⊕ ''A''<sub>''n''</sub>, and | |||
*::<math>p_k \circ i_l</math> | |||
*:is the identity morphism of ''A''<sub>''k''</sub> in the case ''l=k'', and is the zero map otherwise. | |||
== Internal direct sum == | |||
<!-- linked from redirects [[Complementary subspace]] and [[Complementary subspaces]] --> | |||
{{see also|Internal direct product}} | |||
Suppose ''M'' is some ''R''-module, and ''M''<sub>''i''</sub> is a [[submodule]] of ''M'' for every ''i'' in ''I''. If every ''x'' in ''M'' can be written in one and only one way as a sum of finitely many elements of the ''M''<sub>''i''</sub>, then we say that ''M'' is the '''internal direct sum''' of the submodules ''M''<sub>''i''</sub> {{harv|Halmos|1974|loc=§18}}. In this case, ''M'' is naturally isomorphic to the (external) direct sum of the ''M''<sub>''i''</sub> as defined above {{harv|Adamson|1972|loc=p.61}}. | |||
A submodule ''N'' of ''M'' is a '''direct summand''' of ''M'' if there exists some other submodule ''N′'' of ''M'' such that ''M'' is the ''internal'' direct sum of ''N'' and ''N′''. In this case, ''N'' and ''N′'' are '''complementary subspaces'''. | |||
== Universal property == | |||
In the language of [[category theory]], the direct sum is a [[coproduct]] and hence a [[limit (category theory)|colimit]] in the category of left ''R''-modules, which means that it is characterized by the following [[universal property]]. For every ''i'' in ''I'', consider the ''natural embedding'' | |||
:<math>j_i : M_i \rightarrow \bigoplus_{k \in I} M_k</math> | |||
which sends the elements of ''M''<sub>''i''</sub> to those functions which are zero for all arguments but ''i''. If ''f''<sub>''i''</sub> : ''M''<sub>''i''</sub> → ''M'' are arbitrary ''R''-linear maps for every ''i'', then there exists precisely one ''R''-linear map | |||
:<math>f : \bigoplus_{i \in I} M_i \rightarrow M</math> | |||
such that ''f'' o ''j<sub>i</sub>'' = ''f''<sub>''i''</sub> for all ''i''. | |||
Dually, the [[direct product]] is the [[Product (category theory)|product]]. | |||
== Grothendieck group == | |||
The direct sum gives a collection of objects the structure of a commutative [[monoid]], in that the addition of objects is defined, but not subtraction. In fact, subtraction can be defined, and every commutative monoid can be extended to an [[abelian group]]. This extension is known as the [[Grothendieck group]]. The extension is done by defining equivalence classes of pairs of objects, which allows certain pairs to be treated as inverses. The construction, detailed in the article on the Grothendieck group, is "universal", in that it has the [[universal property]] of being unique, and homomorphic to any other embedding of an abelian monoid in an abelian group. | |||
== Direct sum of modules with additional structure == | |||
If the modules we are considering carry some additional structure (e.g. a [[norm (mathematics)|norm]] or an [[inner product]]), then the direct sum of the modules can often be made to carry this additional structure, as well. In this case, we obtain the [[coproduct]] in the appropriate [[category (category theory)|category]] of all objects carrying the additional structure. Three prominent examples occur for [[algebra over a field|algebras over a field]], [[Banach space]]s and [[Hilbert space]]s. | |||
===Direct sum of algebras=== | |||
A direct sum of [[algebra over a field|algebras]] ''X'' and ''Y'' is the direct sum as vector spaces, with product | |||
:<math>(x_1 + y_1) (x_2 + y_2) = (x_1 x_2 + y_1 y_2) .</math> | |||
Consider these classical examples: | |||
:<math>R \oplus R </math> is [[ring isomorphism|ring isomorphic]] to [[split-complex number]]s, also used in [[interval analysis]]. | |||
:<math>C \oplus C </math> is the algebra of [[tessarine]]s introduced by [[James Cockle (lawyer)|James Cockle]] in 1848. | |||
:<math> H \oplus H </math>, called the [[split-biquaternion]]s, was introduced by [[William Kingdon Clifford]] in 1873. | |||
[[Joseph Wedderburn]] exploited the concept of a direct sum of algebras in his classification of [[hypercomplex number]]s. See his ''Lectures on Matrices'' (1934), page 151. | |||
Wedderburn makes clear the distinction between a direct sum and a direct product of algebras: For the direct sum the field of scalars acts jointly on both parts: <math>\lambda (x \oplus y) = \lambda x \oplus \lambda y</math> while for the direct product a scalar factor may be collected alternately with the parts, but not both:<math>\lambda (x,y) = (\lambda x, y) = (x, \lambda y) \!</math>. | |||
[[Ian R. Porteous]] uses the three direct sums above, denoting them <math>^2 R,\ ^2 C,\ ^2 H \!</math>, as rings of scalars in his analysis of ''Clifford Algebras and the Classical Groups'' (1995). These direct sums also arise in the classification of [[composition algebra]]s. | |||
=== Direct sum of Banach spaces === | |||
<span id="Banachspaces"/> | |||
The direct sum of two [[Banach space]]s ''X'' and ''Y'' is the direct sum of ''X'' and ''Y'' considered as vector spaces, with the norm ||(''x'',''y'')|| = ||''x''||<sub>X</sub> + ||''y''||<sub>Y</sub> for all ''x'' in ''X'' and ''y'' in ''Y''. | |||
Generally, if ''X''<sub>''i''</sub> is a collection of Banach spaces, where ''i'' traverses the [[index set]] ''I'', then the direct sum ⊕<sub>''i''∈''I''</sub> ''X''<sub>''i''</sub> is a module consisting of all functions ''x'' [[Domain of a function|defined over]] ''I'' such that ''x''(''i'') ∈ ''X''<sub>''i''</sub> for all ''i'' ∈ ''I'' and | |||
:<math> \sum_{i \in I} \| x(i) \|_{X_i} < \infty. </math> | |||
The norm is given by the sum above. The direct sum with this norm is again a Banach space. | |||
For example, if we take the index set ''I'' = '''N''' and ''X''<sub>''i''</sub> = '''R''', then the direct sum ⊕<sub>''i''∈'''N'''</sub>''X''<sub>''i''</sub> is the space ''l''<sub>1</sub>, which consists of all the sequences (''a''<sub>''i''</sub>) of reals with finite norm ||''a''|| = ∑<sub>''i''</sub> |''a''<sub>''i''</sub>|. | |||
A closed subspace ''A'' of a Banach space ''X'' is '''complemented''' if there is another closed subspace ''B'' of ''X'' such that ''X'' is equal to the internal direct sum <math>A\oplus B</math>. Note that not every closed subspace is complimented, e.g. [[c0 space|''c''<sub>0</sub>]] is not complimented in <math>\ell^\infty</math>. | |||
===Direct sum of modules with bilinear forms=== | |||
Let {(''M''<sub>''i''</sub>,''b''<sub>''i''</sub> : ''i'' ∈ ''I''} be a [[indexed family|family]] indexed by ''I'' of modules equipped with [[bilinear form]]s. The '''orthogonal direct sum''' is the module direct sum with bilinear form ''B'' defined by<ref>{{cite book | first1=J. | last1=Milnor | author1-link=John Milnor| first2=D. | last2=Husemoller | title=Symmetric Bilinear Forms | series=[[Ergebnisse der Mathematik und ihrer Grenzgebiete]] | volume=73 | publisher=[[Springer-Verlag]] | year=1973 | isbn=3-540-06009-X | zbl=0292.10016 | pages=4–5}}</ref> | |||
:<math> B\left({\left({x_i}\right),\left({y_i}\right)}\right) = \sum_{i\in I} b_i\left({x_i,y_i}\right) </math> | |||
in which the summation makes sense even for infinite index sets ''I'' because only finitely many of the terms are non-zero. | |||
===<span id="Hilbertspaces"></span> Direct sum of Hilbert spaces=== | |||
{{further2|[[Positive definite kernel#Direct sum and tensor product]] }} | |||
If finitely many [[Hilbert space]]s ''H''<sub>1</sub>,...,''H''<sub>''n''</sub> are given, one can construct their orthogonal direct sum as above (since they are vector spaces), defining the inner product as: | |||
:<math>\langle (x_1,...,x_n),(y_1,...,y_n) \rangle = \langle x_1,y_1 \rangle +...+ \langle x_n,y_n \rangle. </math> | |||
The resulting direct sum is a Hilbert space which contains the given Hilbert spaces as mutually [[orthogonal]] subspaces. | |||
If infinitely many Hilbert spaces ''H''<sub>''i''</sub> for ''i'' in ''I'' are given, we can carry out the same construction; notice that when defining the inner product, only finitely many summands will be non-zero. However, the result will only be an [[inner product space]] and it will not necessarily be [[completeness (topology)|complete]]. We then define the direct sum of the Hilbert spaces ''H''<sub>''i''</sub> to be the completion of this inner product space. | |||
Alternatively and equivalently, one can define the direct sum of the Hilbert spaces ''H''<sub>''i''</sub> as the space of all functions α with domain ''I'', such that α(''i'') is an element of ''H''<sub>''i''</sub> for every ''i'' in ''I'' and: | |||
:<math>\sum_i \left\| \alpha_{(i)} \right\|^2 < \infty.</math> | |||
The inner product of two such function α and β is then defined as: | |||
:<math>\langle\alpha,\beta\rangle=\sum_i \langle \alpha_i,\beta_i \rangle.</math> | |||
This space is complete and we get a Hilbert space. | |||
For example, if we take the index set ''I'' = '''N''' and ''X''<sub>''i''</sub> = '''R''', then the direct sum ⊕<sub>''i''∈'''N'''</sub> ''X''<sub>''i''</sub> is the space ''l''<sub>2</sub>, which consists of all the sequences (''a''<sub>''i''</sub>) of reals with finite norm <math>\left\| a \right\| = \sqrt{\sum_i \left\| a_i \right\|^2}</math>. Comparing this with the example for Banach spaces, we see that the Banach space direct sum and the Hilbert space direct sum are not necessarily the same. But if there are only finitely many summands, then the Banach space direct sum is isomorphic to the Hilbert space direct sum, although the norm will be different. | |||
Every Hilbert space is isomorphic to a direct sum of sufficiently many copies of the base field (either '''R''' or '''C'''). This is equivalent to the assertion that every Hilbert space has an orthonormal basis. More generally, every closed subspace of a Hilbert space is complemented: it admits an [[orthogonal complement]]. Conversely, the [[Lindenstrauss–Tzafriri theorem]] asserts that if every closed subspace of a Banach space is complemented, then the Banach space is isomorphic (topologically) to a Hilbert space. | |||
== See also == | |||
* [[Biproduct]] | |||
* [[Indecomposable module]] | |||
* [[Jordan–Hölder theorem]] | |||
* [[Krull–Schmidt theorem]] | |||
* [[Split exact sequence]] | |||
== References == | |||
{{reflist}} | |||
* {{Citation | author= Iain T. Adamson | title=Elementary rings and modules | series=University Mathematical Texts | publisher=Oliver and Boyd | year=1972 | isbn=0-05-002192-3 }} | |||
* {{citation|first = Nicolas|last=Bourbaki|authorlink=Nicolas Bourbaki | title = Elements of mathematics, Algebra I| publisher = Springer-Verlag | year = 1989|isbn=3-540-64243-9}}. | |||
* {{citation|first1=David S.|last1=Dummit|first2=Richard M.|last2=Foote|title=Abstract algebra|publisher=Prentice Hall, Inc.|publication-place=Englewood Cliffs, NJ|year=1991|isbn=0-13-004771-6}}. | |||
* {{citation|authorlink=Paul Halmos|first=Paul|last=Halmos|title=Finite dimensional vector spaces|year=1974|publisher=Springer|isbn=0-387-90093-4}} | |||
* {{citation|first1=S.|last1=Mac Lane|authorlink1=Saunders Mac Lane|authorlink2=Garrett Birkhoff|last2=Birkhoff|first2=G.|title=Algebra|publisher=AMS Chelsea|year=1999|isbn=0-8218-1646-2}}. | |||
{{DEFAULTSORT:Direct Sum Of Modules}} | |||
[[Category:Linear algebra]] | |||
[[Category:Module theory]] |
Revision as of 21:28, 2 February 2014
Template:Mergefrom 28 year-old Painting Investments Worker Truman from Regina, usually spends time with pastimes for instance interior design, property developers in new launch ec Singapore and writing. Last month just traveled to City of the Renaissance.
In abstract algebra, the direct sum is a construction which combines several modules into a new, larger module. The direct sum of modules is the smallest module which contains the given modules as submodules with no "unnecessary" constraints, making it an example of a coproduct. Contrast with the direct product, which is the dual notion.
The most familiar examples of this construction occur when considering vector spaces (modules over a field) and abelian groups (modules over the ring Z of integers). The construction may also be extended to cover Banach spaces and Hilbert spaces.
Construction for vector spaces and abelian groups
We give the construction first in these two cases, under the assumption that we have only two objects. Then we generalise to an arbitrary family of arbitrary modules. The key elements of the general construction are more clearly identified by considering these two cases in depth.
Construction for two vector spaces
Suppose V and W are vector spaces over the field K. The cartesian product V × W can be given the structure of a vector space over K Template:Harv by defining the operations componentwise:
- (v1, w1) + (v2, w2) = (v1 + v2, w1 + w2)
- α (v, w) = (α v, α w)
for v, v1, v2 ∈ V, w, w1, w2 ∈ W, and α ∈ K.
The resulting vector space is called the direct sum of V and W and is usually denoted by a plus symbol inside a circle:
It is customary to write the elements of an ordered sum not as ordered pairs (v, w), but as a sum v + w.
The subspace V × {0} of V ⊕ W is isomorphic to V and is often identified with V; similarly for {0} × W and W. (See internal direct sum below.) With this identification, every element of V ⊕ W can be written in one and only one way as the sum of an element of V and an element of W. The dimension of V ⊕ W is equal to the sum of the dimensions of V and W.
This construction readily generalises to any finite number of vector spaces.
Construction for two abelian groups
For abelian groups G and H which are written additively, the direct product of G and H is also called a direct sum Template:Harv. Thus the cartesian product G × H is equipped with the structure of an abelian group by defining the operations componentwise:
- (g1, h1) + (g2, h2) = (g1 + g2, h1 + h2)
for g1, g2 in G, and h1, h2 in H.
Integral multiples are similarly defined componentwise by
- n(g, h) = (ng, nh)
for g in G, h in H, and n an integer. This parallels the extension of the scalar product of vector spaces to the direct sum above.
The resulting abelian group is called the direct sum of G and H and is usually denoted by a plus symbol inside a circle:
It is customary to write the elements of an ordered sum not as ordered pairs (g, h), but as a sum g + h.
The subgroup G × {0} of G ⊕ H is isomorphic to G and is often identified with G; similarly for {0} × H and H. (See internal direct sum below.) With this identification, it is true that every element of G ⊕ H can be written in one and only one way as the sum of an element of G and an element of H. The rank of G ⊕ H is equal to the sum of the ranks of G and H.
This construction readily generalises to any finite number of abelian groups.
Construction for an arbitrary family of modules
One should notice a clear similarity between the definitions of the direct sum of two vector spaces and of two abelian groups. In fact, each is a special case of the construction of the direct sum of two modules. Additionally, by modifying the definition one can accommodate the direct sum of an infinite family of modules. The precise definition is as follows Template:Harv.
Let R be a ring, and {Mi : i ∈ I} a family of left R-modules indexed by the set I. The direct sum of {Mi} is then defined to be the set of all sequences where and for cofinitely many indices i. (The direct product is analogous but the indices do not need to cofinitely vanish.)
It can also be defined as functions α from I to the disjoint union of the modules Mi such that α(i) ∈ Mi for all i ∈ I and α(i) = 0 for cofinitely many indices i. These functions can equivalently be regarded as finitely supported sections of the fiber bundle over the index set I, with the fiber over being .
This set inherits the module structure via component-wise addition and scalar multiplication. Explicitly, two such sequences (or functions) α and β can be added by writing for all i (note that this is again zero for all but finitely many indices), and such a function can be multiplied with an element r from R by defining for all i. In this way, the direct sum becomes a left R-module, and it is denoted
It is customary to write the sequence as a sum . Sometimes a primed summation is used to indicate that cofinitely many of the terms are zero.
Properties
- The direct sum is a submodule of the direct product of the modules Mi Template:Harv. The direct product is the set of all functions α from I to the disjoint union of the modules Mi with α(i)∈Mi, but not necessarily vanishing for all but finitely many i. If the index set I is finite, then the direct sum and the direct product are equal.
- Each of the modules Mi may be identified with the submodule of the direct sum consisting of those functions which vanish on all indices different from i. With these identifications, every element x of the direct sum can be written in one and only one way as a sum of finitely many elements from the modules Mi.
- If the Mi are actually vector spaces, then the dimension of the direct sum is equal to the sum of the dimensions of the Mi. The same is true for the rank of abelian groups and the length of modules.
- Every vector space over the field K is isomorphic to a direct sum of sufficiently many copies of K, so in a sense only these direct sums have to be considered. This is not true for modules over arbitrary rings.
- The tensor product distributes over direct sums in the following sense: if N is some right R-module, then the direct sum of the tensor products of N with Mi (which are abelian groups) is naturally isomorphic to the tensor product of N with the direct sum of the Mi.
- Direct sums are also commutative and associative (up to isomorphism), meaning that it doesn't matter in which order one forms the direct sum.
- The group of R-linear homomorphisms from the direct sum to some left R-module L is naturally isomorphic to the direct product of the groups of R-linear homomorphisms from Mi to L:
- Indeed, there is clearly a homomorphism τ from the left hand side to the right hand side, where τ(θ)(i) is the R-linear homomorphism sending x∈Mi to θ(x) (using the natural inclusion of Mi into the direct sum). The inverse of the homomorphism τ is defined by
- for any α in the direct sum of the modules Mi. The key point is that the definition of τ−1 makes sense because α(i) is zero for all but finitely many i, and so the sum is finite.
- In particular, the dual vector space of a direct sum of vector spaces is isomorphic to the direct product of the duals of those spaces.
- The finite direct sum of modules is a biproduct: If
Internal direct sum
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Suppose M is some R-module, and Mi is a submodule of M for every i in I. If every x in M can be written in one and only one way as a sum of finitely many elements of the Mi, then we say that M is the internal direct sum of the submodules Mi Template:Harv. In this case, M is naturally isomorphic to the (external) direct sum of the Mi as defined above Template:Harv.
A submodule N of M is a direct summand of M if there exists some other submodule N′ of M such that M is the internal direct sum of N and N′. In this case, N and N′ are complementary subspaces.
Universal property
In the language of category theory, the direct sum is a coproduct and hence a colimit in the category of left R-modules, which means that it is characterized by the following universal property. For every i in I, consider the natural embedding
which sends the elements of Mi to those functions which are zero for all arguments but i. If fi : Mi → M are arbitrary R-linear maps for every i, then there exists precisely one R-linear map
such that f o ji = fi for all i.
Dually, the direct product is the product.
Grothendieck group
The direct sum gives a collection of objects the structure of a commutative monoid, in that the addition of objects is defined, but not subtraction. In fact, subtraction can be defined, and every commutative monoid can be extended to an abelian group. This extension is known as the Grothendieck group. The extension is done by defining equivalence classes of pairs of objects, which allows certain pairs to be treated as inverses. The construction, detailed in the article on the Grothendieck group, is "universal", in that it has the universal property of being unique, and homomorphic to any other embedding of an abelian monoid in an abelian group.
Direct sum of modules with additional structure
If the modules we are considering carry some additional structure (e.g. a norm or an inner product), then the direct sum of the modules can often be made to carry this additional structure, as well. In this case, we obtain the coproduct in the appropriate category of all objects carrying the additional structure. Three prominent examples occur for algebras over a field, Banach spaces and Hilbert spaces.
Direct sum of algebras
A direct sum of algebras X and Y is the direct sum as vector spaces, with product
Consider these classical examples:
- is ring isomorphic to split-complex numbers, also used in interval analysis.
- is the algebra of tessarines introduced by James Cockle in 1848.
- , called the split-biquaternions, was introduced by William Kingdon Clifford in 1873.
Joseph Wedderburn exploited the concept of a direct sum of algebras in his classification of hypercomplex numbers. See his Lectures on Matrices (1934), page 151. Wedderburn makes clear the distinction between a direct sum and a direct product of algebras: For the direct sum the field of scalars acts jointly on both parts: while for the direct product a scalar factor may be collected alternately with the parts, but not both:.
Ian R. Porteous uses the three direct sums above, denoting them , as rings of scalars in his analysis of Clifford Algebras and the Classical Groups (1995). These direct sums also arise in the classification of composition algebras.
Direct sum of Banach spaces
The direct sum of two Banach spaces X and Y is the direct sum of X and Y considered as vector spaces, with the norm ||(x,y)|| = ||x||X + ||y||Y for all x in X and y in Y.
Generally, if Xi is a collection of Banach spaces, where i traverses the index set I, then the direct sum ⊕i∈I Xi is a module consisting of all functions x defined over I such that x(i) ∈ Xi for all i ∈ I and
The norm is given by the sum above. The direct sum with this norm is again a Banach space.
For example, if we take the index set I = N and Xi = R, then the direct sum ⊕i∈NXi is the space l1, which consists of all the sequences (ai) of reals with finite norm ||a|| = ∑i |ai|.
A closed subspace A of a Banach space X is complemented if there is another closed subspace B of X such that X is equal to the internal direct sum . Note that not every closed subspace is complimented, e.g. c0 is not complimented in .
Direct sum of modules with bilinear forms
Let {(Mi,bi : i ∈ I} be a family indexed by I of modules equipped with bilinear forms. The orthogonal direct sum is the module direct sum with bilinear form B defined by[1]
in which the summation makes sense even for infinite index sets I because only finitely many of the terms are non-zero.
Direct sum of Hilbert spaces
If finitely many Hilbert spaces H1,...,Hn are given, one can construct their orthogonal direct sum as above (since they are vector spaces), defining the inner product as:
The resulting direct sum is a Hilbert space which contains the given Hilbert spaces as mutually orthogonal subspaces.
If infinitely many Hilbert spaces Hi for i in I are given, we can carry out the same construction; notice that when defining the inner product, only finitely many summands will be non-zero. However, the result will only be an inner product space and it will not necessarily be complete. We then define the direct sum of the Hilbert spaces Hi to be the completion of this inner product space.
Alternatively and equivalently, one can define the direct sum of the Hilbert spaces Hi as the space of all functions α with domain I, such that α(i) is an element of Hi for every i in I and:
The inner product of two such function α and β is then defined as:
This space is complete and we get a Hilbert space.
For example, if we take the index set I = N and Xi = R, then the direct sum ⊕i∈N Xi is the space l2, which consists of all the sequences (ai) of reals with finite norm . Comparing this with the example for Banach spaces, we see that the Banach space direct sum and the Hilbert space direct sum are not necessarily the same. But if there are only finitely many summands, then the Banach space direct sum is isomorphic to the Hilbert space direct sum, although the norm will be different.
Every Hilbert space is isomorphic to a direct sum of sufficiently many copies of the base field (either R or C). This is equivalent to the assertion that every Hilbert space has an orthonormal basis. More generally, every closed subspace of a Hilbert space is complemented: it admits an orthogonal complement. Conversely, the Lindenstrauss–Tzafriri theorem asserts that if every closed subspace of a Banach space is complemented, then the Banach space is isomorphic (topologically) to a Hilbert space.
See also
References
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- Many property agents need to declare for the PIC grant in Singapore. However, not all of them know find out how to do the correct process for getting this PIC scheme from the IRAS. There are a number of steps that you need to do before your software can be approved.
Naturally, you will have to pay a safety deposit and that is usually one month rent for annually of the settlement. That is the place your good religion deposit will likely be taken into account and will kind part or all of your security deposit. Anticipate to have a proportionate amount deducted out of your deposit if something is discovered to be damaged if you move out. It's best to you'll want to test the inventory drawn up by the owner, which can detail all objects in the property and their condition. If you happen to fail to notice any harm not already mentioned within the inventory before transferring in, you danger having to pay for it yourself.
In case you are in search of an actual estate or Singapore property agent on-line, you simply should belief your intuition. It's because you do not know which agent is nice and which agent will not be. Carry out research on several brokers by looking out the internet. As soon as if you end up positive that a selected agent is dependable and reliable, you can choose to utilize his partnerise in finding you a home in Singapore. Most of the time, a property agent is taken into account to be good if he or she locations the contact data on his website. This may mean that the agent does not mind you calling them and asking them any questions relating to new properties in singapore in Singapore. After chatting with them you too can see them in their office after taking an appointment.
Have handed an trade examination i.e Widespread Examination for House Brokers (CEHA) or Actual Property Agency (REA) examination, or equal; Exclusive brokers are extra keen to share listing information thus making certain the widest doable coverage inside the real estate community via Multiple Listings and Networking. Accepting a severe provide is simpler since your agent is totally conscious of all advertising activity related with your property. This reduces your having to check with a number of agents for some other offers. Price control is easily achieved. Paint work in good restore-discuss with your Property Marketing consultant if main works are still to be done. Softening in residential property prices proceed, led by 2.8 per cent decline within the index for Remainder of Central Region
Once you place down the one per cent choice price to carry down a non-public property, it's important to accept its situation as it is whenever you move in – faulty air-con, choked rest room and all. Get round this by asking your agent to incorporate a ultimate inspection clause within the possibility-to-buy letter. HDB flat patrons routinely take pleasure in this security net. "There's a ultimate inspection of the property two days before the completion of all HDB transactions. If the air-con is defective, you can request the seller to repair it," says Kelvin.
15.6.1 As the agent is an intermediary, generally, as soon as the principal and third party are introduced right into a contractual relationship, the agent drops out of the image, subject to any problems with remuneration or indemnification that he could have against the principal, and extra exceptionally, against the third occasion. Generally, agents are entitled to be indemnified for all liabilities reasonably incurred within the execution of the brokers´ authority.
To achieve the very best outcomes, you must be always updated on market situations, including past transaction information and reliable projections. You could review and examine comparable homes that are currently available in the market, especially these which have been sold or not bought up to now six months. You'll be able to see a pattern of such report by clicking here It's essential to defend yourself in opposition to unscrupulous patrons. They are often very skilled in using highly unethical and manipulative techniques to try and lure you into a lure. That you must also protect your self, your loved ones, and personal belongings as you'll be serving many strangers in your home. Sign a listing itemizing of all of the objects provided by the proprietor, together with their situation. HSR Prime Recruiter 2010 - Many property agents need to declare for the PIC grant in Singapore. However, not all of them know find out how to do the correct process for getting this PIC scheme from the IRAS. There are a number of steps that you need to do before your software can be approved.
Naturally, you will have to pay a safety deposit and that is usually one month rent for annually of the settlement. That is the place your good religion deposit will likely be taken into account and will kind part or all of your security deposit. Anticipate to have a proportionate amount deducted out of your deposit if something is discovered to be damaged if you move out. It's best to you'll want to test the inventory drawn up by the owner, which can detail all objects in the property and their condition. If you happen to fail to notice any harm not already mentioned within the inventory before transferring in, you danger having to pay for it yourself.
In case you are in search of an actual estate or Singapore property agent on-line, you simply should belief your intuition. It's because you do not know which agent is nice and which agent will not be. Carry out research on several brokers by looking out the internet. As soon as if you end up positive that a selected agent is dependable and reliable, you can choose to utilize his partnerise in finding you a home in Singapore. Most of the time, a property agent is taken into account to be good if he or she locations the contact data on his website. This may mean that the agent does not mind you calling them and asking them any questions relating to new properties in singapore in Singapore. After chatting with them you too can see them in their office after taking an appointment.
Have handed an trade examination i.e Widespread Examination for House Brokers (CEHA) or Actual Property Agency (REA) examination, or equal; Exclusive brokers are extra keen to share listing information thus making certain the widest doable coverage inside the real estate community via Multiple Listings and Networking. Accepting a severe provide is simpler since your agent is totally conscious of all advertising activity related with your property. This reduces your having to check with a number of agents for some other offers. Price control is easily achieved. Paint work in good restore-discuss with your Property Marketing consultant if main works are still to be done. Softening in residential property prices proceed, led by 2.8 per cent decline within the index for Remainder of Central Region
Once you place down the one per cent choice price to carry down a non-public property, it's important to accept its situation as it is whenever you move in – faulty air-con, choked rest room and all. Get round this by asking your agent to incorporate a ultimate inspection clause within the possibility-to-buy letter. HDB flat patrons routinely take pleasure in this security net. "There's a ultimate inspection of the property two days before the completion of all HDB transactions. If the air-con is defective, you can request the seller to repair it," says Kelvin.
15.6.1 As the agent is an intermediary, generally, as soon as the principal and third party are introduced right into a contractual relationship, the agent drops out of the image, subject to any problems with remuneration or indemnification that he could have against the principal, and extra exceptionally, against the third occasion. Generally, agents are entitled to be indemnified for all liabilities reasonably incurred within the execution of the brokers´ authority.
To achieve the very best outcomes, you must be always updated on market situations, including past transaction information and reliable projections. You could review and examine comparable homes that are currently available in the market, especially these which have been sold or not bought up to now six months. You'll be able to see a pattern of such report by clicking here It's essential to defend yourself in opposition to unscrupulous patrons. They are often very skilled in using highly unethical and manipulative techniques to try and lure you into a lure. That you must also protect your self, your loved ones, and personal belongings as you'll be serving many strangers in your home. Sign a listing itemizing of all of the objects provided by the proprietor, together with their situation. HSR Prime Recruiter 2010. - Many property agents need to declare for the PIC grant in Singapore. However, not all of them know find out how to do the correct process for getting this PIC scheme from the IRAS. There are a number of steps that you need to do before your software can be approved.
Naturally, you will have to pay a safety deposit and that is usually one month rent for annually of the settlement. That is the place your good religion deposit will likely be taken into account and will kind part or all of your security deposit. Anticipate to have a proportionate amount deducted out of your deposit if something is discovered to be damaged if you move out. It's best to you'll want to test the inventory drawn up by the owner, which can detail all objects in the property and their condition. If you happen to fail to notice any harm not already mentioned within the inventory before transferring in, you danger having to pay for it yourself.
In case you are in search of an actual estate or Singapore property agent on-line, you simply should belief your intuition. It's because you do not know which agent is nice and which agent will not be. Carry out research on several brokers by looking out the internet. As soon as if you end up positive that a selected agent is dependable and reliable, you can choose to utilize his partnerise in finding you a home in Singapore. Most of the time, a property agent is taken into account to be good if he or she locations the contact data on his website. This may mean that the agent does not mind you calling them and asking them any questions relating to new properties in singapore in Singapore. After chatting with them you too can see them in their office after taking an appointment.
Have handed an trade examination i.e Widespread Examination for House Brokers (CEHA) or Actual Property Agency (REA) examination, or equal; Exclusive brokers are extra keen to share listing information thus making certain the widest doable coverage inside the real estate community via Multiple Listings and Networking. Accepting a severe provide is simpler since your agent is totally conscious of all advertising activity related with your property. This reduces your having to check with a number of agents for some other offers. Price control is easily achieved. Paint work in good restore-discuss with your Property Marketing consultant if main works are still to be done. Softening in residential property prices proceed, led by 2.8 per cent decline within the index for Remainder of Central Region
Once you place down the one per cent choice price to carry down a non-public property, it's important to accept its situation as it is whenever you move in – faulty air-con, choked rest room and all. Get round this by asking your agent to incorporate a ultimate inspection clause within the possibility-to-buy letter. HDB flat patrons routinely take pleasure in this security net. "There's a ultimate inspection of the property two days before the completion of all HDB transactions. If the air-con is defective, you can request the seller to repair it," says Kelvin.
15.6.1 As the agent is an intermediary, generally, as soon as the principal and third party are introduced right into a contractual relationship, the agent drops out of the image, subject to any problems with remuneration or indemnification that he could have against the principal, and extra exceptionally, against the third occasion. Generally, agents are entitled to be indemnified for all liabilities reasonably incurred within the execution of the brokers´ authority.
To achieve the very best outcomes, you must be always updated on market situations, including past transaction information and reliable projections. You could review and examine comparable homes that are currently available in the market, especially these which have been sold or not bought up to now six months. You'll be able to see a pattern of such report by clicking here It's essential to defend yourself in opposition to unscrupulous patrons. They are often very skilled in using highly unethical and manipulative techniques to try and lure you into a lure. That you must also protect your self, your loved ones, and personal belongings as you'll be serving many strangers in your home. Sign a listing itemizing of all of the objects provided by the proprietor, together with their situation. HSR Prime Recruiter 2010. - Many property agents need to declare for the PIC grant in Singapore. However, not all of them know find out how to do the correct process for getting this PIC scheme from the IRAS. There are a number of steps that you need to do before your software can be approved.
Naturally, you will have to pay a safety deposit and that is usually one month rent for annually of the settlement. That is the place your good religion deposit will likely be taken into account and will kind part or all of your security deposit. Anticipate to have a proportionate amount deducted out of your deposit if something is discovered to be damaged if you move out. It's best to you'll want to test the inventory drawn up by the owner, which can detail all objects in the property and their condition. If you happen to fail to notice any harm not already mentioned within the inventory before transferring in, you danger having to pay for it yourself.
In case you are in search of an actual estate or Singapore property agent on-line, you simply should belief your intuition. It's because you do not know which agent is nice and which agent will not be. Carry out research on several brokers by looking out the internet. As soon as if you end up positive that a selected agent is dependable and reliable, you can choose to utilize his partnerise in finding you a home in Singapore. Most of the time, a property agent is taken into account to be good if he or she locations the contact data on his website. This may mean that the agent does not mind you calling them and asking them any questions relating to new properties in singapore in Singapore. After chatting with them you too can see them in their office after taking an appointment.
Have handed an trade examination i.e Widespread Examination for House Brokers (CEHA) or Actual Property Agency (REA) examination, or equal; Exclusive brokers are extra keen to share listing information thus making certain the widest doable coverage inside the real estate community via Multiple Listings and Networking. Accepting a severe provide is simpler since your agent is totally conscious of all advertising activity related with your property. This reduces your having to check with a number of agents for some other offers. Price control is easily achieved. Paint work in good restore-discuss with your Property Marketing consultant if main works are still to be done. Softening in residential property prices proceed, led by 2.8 per cent decline within the index for Remainder of Central Region
Once you place down the one per cent choice price to carry down a non-public property, it's important to accept its situation as it is whenever you move in – faulty air-con, choked rest room and all. Get round this by asking your agent to incorporate a ultimate inspection clause within the possibility-to-buy letter. HDB flat patrons routinely take pleasure in this security net. "There's a ultimate inspection of the property two days before the completion of all HDB transactions. If the air-con is defective, you can request the seller to repair it," says Kelvin.
15.6.1 As the agent is an intermediary, generally, as soon as the principal and third party are introduced right into a contractual relationship, the agent drops out of the image, subject to any problems with remuneration or indemnification that he could have against the principal, and extra exceptionally, against the third occasion. Generally, agents are entitled to be indemnified for all liabilities reasonably incurred within the execution of the brokers´ authority.
To achieve the very best outcomes, you must be always updated on market situations, including past transaction information and reliable projections. You could review and examine comparable homes that are currently available in the market, especially these which have been sold or not bought up to now six months. You'll be able to see a pattern of such report by clicking here It's essential to defend yourself in opposition to unscrupulous patrons. They are often very skilled in using highly unethical and manipulative techniques to try and lure you into a lure. That you must also protect your self, your loved ones, and personal belongings as you'll be serving many strangers in your home. Sign a listing itemizing of all of the objects provided by the proprietor, together with their situation. HSR Prime Recruiter 2010 - Many property agents need to declare for the PIC grant in Singapore. However, not all of them know find out how to do the correct process for getting this PIC scheme from the IRAS. There are a number of steps that you need to do before your software can be approved.
Naturally, you will have to pay a safety deposit and that is usually one month rent for annually of the settlement. That is the place your good religion deposit will likely be taken into account and will kind part or all of your security deposit. Anticipate to have a proportionate amount deducted out of your deposit if something is discovered to be damaged if you move out. It's best to you'll want to test the inventory drawn up by the owner, which can detail all objects in the property and their condition. If you happen to fail to notice any harm not already mentioned within the inventory before transferring in, you danger having to pay for it yourself.
In case you are in search of an actual estate or Singapore property agent on-line, you simply should belief your intuition. It's because you do not know which agent is nice and which agent will not be. Carry out research on several brokers by looking out the internet. As soon as if you end up positive that a selected agent is dependable and reliable, you can choose to utilize his partnerise in finding you a home in Singapore. Most of the time, a property agent is taken into account to be good if he or she locations the contact data on his website. This may mean that the agent does not mind you calling them and asking them any questions relating to new properties in singapore in Singapore. After chatting with them you too can see them in their office after taking an appointment.
Have handed an trade examination i.e Widespread Examination for House Brokers (CEHA) or Actual Property Agency (REA) examination, or equal; Exclusive brokers are extra keen to share listing information thus making certain the widest doable coverage inside the real estate community via Multiple Listings and Networking. Accepting a severe provide is simpler since your agent is totally conscious of all advertising activity related with your property. This reduces your having to check with a number of agents for some other offers. Price control is easily achieved. Paint work in good restore-discuss with your Property Marketing consultant if main works are still to be done. Softening in residential property prices proceed, led by 2.8 per cent decline within the index for Remainder of Central Region
Once you place down the one per cent choice price to carry down a non-public property, it's important to accept its situation as it is whenever you move in – faulty air-con, choked rest room and all. Get round this by asking your agent to incorporate a ultimate inspection clause within the possibility-to-buy letter. HDB flat patrons routinely take pleasure in this security net. "There's a ultimate inspection of the property two days before the completion of all HDB transactions. If the air-con is defective, you can request the seller to repair it," says Kelvin.
15.6.1 As the agent is an intermediary, generally, as soon as the principal and third party are introduced right into a contractual relationship, the agent drops out of the image, subject to any problems with remuneration or indemnification that he could have against the principal, and extra exceptionally, against the third occasion. Generally, agents are entitled to be indemnified for all liabilities reasonably incurred within the execution of the brokers´ authority.
To achieve the very best outcomes, you must be always updated on market situations, including past transaction information and reliable projections. You could review and examine comparable homes that are currently available in the market, especially these which have been sold or not bought up to now six months. You'll be able to see a pattern of such report by clicking here It's essential to defend yourself in opposition to unscrupulous patrons. They are often very skilled in using highly unethical and manipulative techniques to try and lure you into a lure. That you must also protect your self, your loved ones, and personal belongings as you'll be serving many strangers in your home. Sign a listing itemizing of all of the objects provided by the proprietor, together with their situation. HSR Prime Recruiter 2010.
- ↑ 20 year-old Real Estate Agent Rusty from Saint-Paul, has hobbies and interests which includes monopoly, property developers in singapore and poker. Will soon undertake a contiki trip that may include going to the Lower Valley of the Omo.
My blog: http://www.primaboinca.com/view_profile.php?userid=5889534