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In [[mathematics]], a '''Witt vector''' is an [[infinite sequence]] of elements of a [[commutative ring]]. [[Ernst Witt]] showed how to put a ring [[mathematical structure|structure]] on the set of Witt vectors, in such a way that the ring of Witt vectors over the finite field of order ''p'' is the ring of [[p-adic integer|''p''-adic integers]].
 
==Motivation==
We basically want to derive the ring <math>p</math>-adic integers <math>\mathbb{Z}_p</math> from the finite field with <math>p</math> elements, <math>\mathbb{F}_p</math>, by some '''general construction'''.
 
A <math>p</math>-adic integer is a sequence <math>(n_0,n_1,...)</math> with <math>n_i\in\mathbb{Z}/p^{(n+1)}\mathbb{Z}</math>,such that <math>n_i\equiv n_j\mod p^i</math> if <math>i<j</math>. They can be expanded as a [[power series]] in <math>p</math>:
<math>a_0 + a_1 p^1 + a_2 p^2 + ...</math>, where the <math>a</math>'s are usually taken from the set <math>\{0, 1, 2, ..., p-1\}</math> (The equation is happening in <math>\mathbb{Z}_p</math>, with <math>a_i</math> and <math>p^j</math> all images from <math>\mathbb{Z}</math> to <math>\mathbb{Z}_p</math>). '''Set-theoretically''' it is <math>\mathbb{F}_p</math>. But <math>\mathbb{Z}_p</math> is not isomorphic to <math>\prod_{\mathbb{N}}\mathbb{F}_p</math>. If we denote <math>a+b=c</math>, then the addition should instead be:
:<math>
c_0 \equiv a_0+b_0 \mod p
</math>
:<math>
c_0+c_1 p\equiv a_0+a_1 p+b_0+b_1 p \mod p^2
</math>
:<math>
c_0+c_1 p+c_2 p^2 \equiv a_0+a_1 p+a_2 p^2+b_0+b_1 p+b_2 p^2 \mod p^3
</math>
But we lack some properties of the coefficients to produce a general formula.
 
Luckily, there is an alternative subset of <math>\mathbb{Z}_p</math> which can work as the coefficient set. This is the [[Teichmüller character|Teichmüller representatives]] of elements of <math>\mathbb{F}_p</math>. Without <math>0</math> they form a subgroup of <math>\mathbb{Z}_p^*</math>, identified with <math>\mathbb{F}_p^*</math> through the [[Teichmüller character]] <math>\omega:\mathbb{F}_p^*\rightarrow\mathbb{Z}_p^*</math>. Note this is '''not''' additive, as the sum need not be a representative. Despite this, if <math>\omega(k)=\omega(i)+\omega(j)\mod p</math> in <math>\mathbb{Z}_p</math>, then <math>i+j=k</math> in <math>\mathbb{F}_p</math>. This is conceptually justified by <math>m\circ \omega=\mathrm{id}_{\mathbb{F}_p}</math> if we denote <math>m:\mathbb{Z}_p\rightarrow\mathbb{Z}_p/p\mathbb{Z}_p\cong\mathbb{F}_p</math>.
 
They are explicitly calculated as roots of <math>x^{p-1}-1=0</math> through [[Hensel's lemma#Hensel Lifting|Hensel lifting]]. For example, in <math>\mathbb{Z}_3</math>, to calculate the representative of <math>2</math>, you first find the unique solution of <math>x^{2}-1=0</math> in <math>\mathbb{Z}/9\mathbb{Z}</math> with <math>x\equiv 2\mod 3</math>; You get <math>8</math>, then repeat it in <math>\mathbb{Z}/27\mathbb{Z}</math>, with conditions <math>x^{2}-1=0</math> and <math>x\equiv 2\mod 9</math>; This time it is <math>26</math>, and so on. The existence of lift in each step is guaranteed by <math>(x^{p-1}-1,(p-1)x^{p-2})=1</math> in every <math>\mathbb{Z}/p^n\mathbb{Z}</math>.
 
We can also write the representatives as <math>a_{0} + a_{1} p^1 + a_{2} p^2 + ...</math>. Note for every <math>j\in\{0, 1, 2, ..., p-1\}</math>, there is exactly one representative, namely <math>\omega(j)</math>, with <math>a_{0}=j</math>, so we can also expand every <math>p</math>-adic integer as a power series in <math>p</math>, with coefficients from the Teichmüller representatives.
 
Explicitly, if <math>b=a_{0} + a_{1} p^1 + a_{2} p^2 + ...</math>, then <math>b-\omega(a_0)=a'_{1} p^1 + a'_{2} p^2 + ...</math>. Then you subtract <math>\omega(a'_1)p</math> and proceed similarly. Note the coefficients you get most probably differ from the <math>a_i</math>'s, except the first one.
 
This time we have additional properties of the coefficients like <math>a_i^p=a_i</math>, so we can make some changes to get a neat formula. Since the Teichmüller character is '''not''' additive, we don't have <math>c_0=a_0+b_0</math> in <math>\mathbb{Z}_p</math>. But it happens in <math>\mathbb{F}_p</math>, as the first congruence implies. So actually <math>c_0^p\equiv (a_0+b_0)^p \mod p^2</math>, thus <math>c_0-a_0-b_0\equiv (a_0+b_0)^p-a_0-b_0\equiv \binom{p}{1} a_0^{p-1}b_0+...+ \binom{p}{1} a_0 b_0^{p-1} \mod p^2</math>. Since <math>\binom{p}{i}</math> is divisible by <math>p</math>, this resolves the <math>p</math>-coefficient problem of <math>c_1</math> and gives <math>c_1\equiv a_1+b_1- a_0^{p-1}b_0-\frac{p-1}{2}a_0^{p-2}b_0^2-...- a_0 b_0^{p-1}\mod p</math>. Note this completely determines <math>c_1</math> by the lift. Moreover, the <math>\mod p</math> indicates that the calculation can actually be done in <math>\mathbb{F}_p</math>, satisfying our basic aim.
 
Now for <math>c_2</math>. It is already very cumbersome at this step. <math>c_1=c_1^p \equiv (a_1+b_1- a_0^{p-1}b_0-\frac{p-1}{2}a_0^{p-2}b_0^2-...- a_0 b_0^{p-1})^p\mod p</math>. As for <math>c_0</math>, a single <math>p</math>th power is not enough: actually we take <math>c_0=c_0^{p^2}\equiv(a_0+b_0)^{p^2}</math>. <math>\binom{p^2}{i}</math> is not always divisible by <math>p^2</math>, but that only happens when <math>i=pd</math>, in which case <math>a^ib^{p^2-i}=a^db^{p-d}</math> combined with similar monomials in <math>c_1^p</math> would make a multiple of <math>p^2</math>.
 
At this step, we see that we are actually working with something like
:<math>
c_0 \equiv a_0+b_0 \mod p
</math>
:<math>
c_0^p+c_1 p\equiv a_0^p+a_1 p+b_0^p+b_1 p \mod p^2
</math>
:<math>
c_0^{p^2}+c_1^p p+c_2 p^2 \equiv a_0^{p^2}+a_1^p p+a_2 p^2+b_0^{p^2}+b_1^p p+b_2 p^2 \mod p^3
</math>
This motivates the definition of Witt vectors.
 
==Construction of Witt rings==
Fix a [[prime number]] ''p''. A '''Witt vector''' over a commutative ring ''R'' is a sequence :<math> (X_0,X_1,X_2,...)</math> of elements of ''R''.  Define the '''Witt polynomials''' <math>W_i</math> by
#<math> W_0=X_0\,</math>
#<math> W_1=X_0^p+pX_1</math>
#<math> W_2=X_0^{p^2}+pX_1^p+p^2X_2</math>
and in general
:<math> W_n=\sum_ip^iX_i^{p^{n-i}}.</math>
 
<math> (W_0,W_1,W_2,...)</math> is called the '''ghost components''' of the Witt vector <math>(X_0,X_1,X_2,...)</math>, and is usually denoted by <math> (X^{(0)},X^{(1)},X^{(2)},...)</math>.
 
Then Witt showed that there is a unique way to make the set of Witt vectors over any commutative ring ''R'' into a ring, called the '''ring of Witt vectors''', such that
*the sum and product are given by polynomials with integral coefficients that do not depend on ''R'', and
*Every Witt polynomial is a homomorphism from the ring of Witt vectors over ''R'' to ''R''.
In other words, if
*<math> (X+Y)_i</math> and <math> (XY)_i</math> are given by polynomials with integral coefficients that do not depend on ''R'', and
*<math> X^{(i)}+Y^{(i)}=(X+Y)^{(i)}</math>, <math> X^{(i)}Y^{(i)}=(XY)^{(i)}</math>.
 
The first few polynomials giving the sum and product of Witt vectors can be written down explicitly. For example,
*<math>(X_0,X_1,...)+(Y_0,Y_1,...)=(X_0+Y_0,X_1+Y_1+(X_0^p+Y_0^p-(X_0+Y_0)^p)/p,...)</math>
*<math>(X_0,X_1,...)\times(Y_0,Y_1,...)=(X_0 Y_0,X_0^p Y_1+X_1 Y_0^p+p X_1 Y_1,...)</math>.
 
==Examples==
*The Witt ring of any commutative ring ''R'' in which ''p'' is invertible is just isomorphic to ''R''<sup>'''N'''</sup> (the product of a countable number of copies of ''R''). In fact the Witt polynomials always give a homomorphism from the ring of Witt vectors to ''R''<sup>'''N'''</sup>, and if ''p'' is invertible this homomorphism is an isomorphism.
*The Witt ring of the [[finite field]] of order ''p'' is the ring of ''p''-adic integers, as is demonstrated above.
*The Witt ring of a finite field of order ''p''<sup>''n''</sup> is the [[Splitting of prime ideals in Galois extensions|unramified extension]] of degree ''n'' of the ring of ''p''-adic integers.
 
==Universal Witt vectors==
The Witt polynomials for different primes ''p'' are special cases of universal Witt polynomials, which can be used to form a universal Witt ring (not depending on a choice of prime ''p'').
Define the universal Witt polynomials ''W''<sub>''n''</sub> for ''n''≥1 by
#<math> W_1=X_1\,</math>
#<math> W_2=X_1^2+2X_2</math>
#<math> W_3=X_1^3+3X_3</math>
#<math> W_4=X_1^{4}+2X_2^2+4X_4</math>
and in general
:<math> W_n=\sum_{d|n}dX_d^{n/d}.</math>
 
Again, <math>(W_1,W_2,W_3,...)</math> is called the '''ghost components''' of the Witt vector <math>(X_1,X_2,X_3,...)</math>, and is usually denoted by <math> (X^{(1)},X^{(2)},X^{(3)},...)</math>.
 
We can use these polynomials to define the ring of universal Witt vectors over any commutative ring ''R'' in much the same way as above (so the universal Witt polynomials are all homomorphisms to the ring ''R'').
 
==Generating Functions==
Later Witt orally stated another approach using generating functions.<ref>{{cite book |last=Lang |first=Serge |title=Algebra |publisher=Springer|edition = 3rd |date=September 19, 2005 |pages=330 |chapter=Chapter VI: Galois Theory |isbn=978-0-387-95385-4}}</ref>
 
===Definition===
Let <math>X</math> be a Witt vector and define
:<math>f_X(t)=\prod_{n\ge 1}(1-X_n t^n)=\sum_{n\ge 0}A_n t^n</math>
For <math>n\ge 1</math> let <math>\mathcal{S}_n</math> denote the collection of subsets of <math>\{1,2,...,n\}</math> whose elements add up to <math>n</math>. Then <math>A_n=\sum_{S\in\mathcal{S}}(-1)^{|S|}\sum_{i\in S}{X_i}</math>.
 
We can get the ghost components by taking the [[logarithmic derivative]]:
:<math>\frac{d}{dt}\log f_X(t)=\sum_{n\ge 1}\frac{d}{dt}(1-X_n t^n)=-\sum_{n\ge 1}\sum_{d\ge 1}\frac{X_n^d t^{nd}}{d}=-\sum_{m\ge 1}\frac{\sum_{d|m}\frac{m}{d}X_{\frac{m}{d}}^d}{m}t^m=-\sum_{m\ge 1}\frac{X^{(m)}t^m}{m}</math>
 
===Sum===
Now we can see <math>f_{Z}(t)=f_X(t) f_Y(t)</math> if <math>Z=X+Y</math>. So that <math>C_n=\sum_{0\le i\le n}A_n B_{n-i}</math> if <math>A_n,B_n,C_n</math> are respective coefficients in the power series for <math>f_X(t),f_Y(t),f_Z(t)</math>. Then <math>Z_n=\sum_{0\le i\le n}A_n B_{n-i}-\sum_{S\in\mathcal{S},S\ne\{n\}}(-1)^{|S|}\sum_{i\in S}{Z_i}</math>. Since <math>A_n</math> is a polynomial in <math>X_1,...,X_n</math> and likely for <math>B_n</math>, we can show by induction that <math>Z_n</math> is a polynomial in <math>X_1,...,X_n,Y_1,...,Y_n</math>.
 
===Product===
If we set <math>W=XY</math> then
:<math>\frac{d}{dt}\log f_W(t)=-\sum_{m\ge 1}\frac{X^{(m)}Y^{(m)}t^m}{m}</math>
But
:<math>\sum_{m\ge 1}\frac{X^{(m)}Y^{(m)}}{m}t^m=\sum_{m\ge 1}\frac{\sum_{d|m}d X_d^{m/d}\sum_{e|m}e Y_e^{m/e}}{m}t^m</math>
Now 3-tuples <math>{m,d,e}</math> with <math>m\in\mathbb{Z}^+,d|m,e|m</math> are in bijection with 3-tuples <math>{d,e,n}</math> with <math>d,e,n\in\mathbb{Z}^+</math>, via <math>n=m/[d,e]</math> (<math>[d,e]</math> is the [[Least common multiple]]), our series becomes
:<math>\sum_{d,e\ge 1}\frac{\frac{d e}{ [d,e]}\sum_{n\ge 1} (X_d^{ [d,e]/d } Y_e^{ [d,e]/e } t^{ [d,e] })^n}{n}</math>
So that
:<math>f_W(t)=\prod_{d,e\ge 1}(1-X_d^{[d,e]/d}Y_e^{[d,e]/e} t^{[d,e]})^{d e/[d,e]}=\sum_{n\ge 0}D_n t^n</math>
where <math>D_n</math>s are polynomials of <math>X_1,...,X_n,Y_1,...,Y_n</math>. So by similar induction, suppose <math>f_W(t)=\prod_{n\ge 1}(1-W_n t^n)</math>, then <math>W_n</math> can be solved as polynomials of <math>X_1,...,X_n,Y_1,...,Y_n</math>.
 
==Ring schemes==
The map taking a commutative ring ''R'' to the ring of Witt vectors over ''R'' (for a fixed prime ''p'') is a [[functor]] from commutative rings to commutative rings, and is also representable, so it can be thought of as a [[ring scheme]], called the '''Witt scheme''',  over Spec('''Z'''). The Witt scheme can be canonically identified with the spectrum of the [[ring of symmetric functions]].
 
Similarly the rings of truncated Witt vectors, and the rings of
universal Witt vectors, correspond to ring schemes, called the '''truncated Witt schemes''' and the '''universal Witt scheme''' .
 
Moreover, the functor taking the commutative ring <math>R</math> to the set <math>R^n</math> is represented by the affine space <math>\mathbb{A}_{\mathbb{Z}}^n</math>, and the ring structure on ''R<sup>n</sup>'' makes <math>\mathbb{A}_{\mathbb{Z}}^n</math> into a ring scheme denoted <math>\underline{\mathcal{O}}^n</math>. From the construction of truncated Witt vectors it follows that their associated ring scheme <math>\mathbb{W}_n</math> is the scheme <math>\mathbb{A}_{\mathbb{Z}}^n</math> with the unique ring structure such that the morphism <math>\mathbb{W}_n\rightarrow \underline{\mathcal{O}}^n</math> given by the Witt polynomials is a morphism of ring schemes.
 
==Commutative unipotent algebraic groups==
Over an [[algebraically closed field]] of characteristic 0, any [[unipotent]] abelian connected [[algebraic group]] is isomorphic to a product of copies of the additive group <math>G_a</math>.
The analogue of this for fields of characteristic ''p'' is false: the truncated Witt schemes are counterexamples. (We make them into algebraic groups by forgetting the multiplication and just using the additive structure.) However these are essentially the only counterexamples: over an algebraically closed field of characteristic ''p'', any [[unipotent]] abelian connected [[algebraic group]] is  
[[Isogeny#Abelian varieties up to isogeny|isogenous]] to a product of truncated Witt group schemes.
 
==See also==
*[[Formal group]]
*[[Artin–Hasse exponential]]
 
==References==
{{reflist}}
*{{springer|authorlink=Dolgachev|first=I.V. |last=Dolgachev|id=Witt_vector|title=Witt vector}}
*{{citation|mr=2553661
|last=Hazewinkel|first= Michiel
|chapter=Witt vectors. I.|title= Handbook of algebra. Vol. 6|pages=319–472,
|publisher= Elsevier/North-Holland|place= Amsterdam|year= 2009|arxiv=0804.3888|isbn=978-0-444-53257-2}}
* {{Citation | last1=Mumford | first1=David | author1-link = David Mumford | title=Lectures on Curves on an Algebraic Surface | publisher=[[Princeton University Press]] | location=Princeton, NJ | series=Annals of Mathematics Studies | isbn=978-0-691-07993-6 | volume=59 | year=}}
* {{Citation | last1=Serre | first1=Jean-Pierre | author1-link = Jean-Pierre Serre | title=Local fields | publisher=[[Springer-Verlag]] | location=Berlin, New York | series=Graduate Texts in Mathematics | isbn=978-0-387-90424-5 | mr=554237 | year=1979 | volume=67}}, section II.6
* {{Citation | last1=Serre | first1=Jean-Pierre | title=Algebraic groups and class fields | publisher=[[Springer-Verlag]] | location=Berlin, New York | series=Graduate Texts in Mathematics | isbn=978-0-387-96648-9 | mr=918564 | year=1988 | volume=117}}
* {{Citation | url=http://www.digizeitschriften.de/main/dms/img/?IDDOC=504725 | last1=Witt | first1=Ernst | author1-link = Ernst Witt | title=Zyklische Körper und Algebren der Characteristik p vom Grad p<sup>n</sup>. Struktur diskret bewerteter perfekter Körper mit vollkommenem Restklassenkörper der Charakteristik p<sup>n</sup> | language=German | year=1936 | journal=Journal für Reine und Angewandte Mathematik | volume=176 | pages=126–140}}
*Greenberg, M. J. (1969), ''Lectures on Forms in Many Variables'', New York and Amsterdam, Benjamin, {{MathSciNet|id=241358}}, ASIN: B0006BX17M
 
[[Category:Ring theory]]
[[Category:Algebraic groups]]
[[Category:Combinatorics on words]]

Revision as of 21:41, 22 August 2013

In mathematics, a Witt vector is an infinite sequence of elements of a commutative ring. Ernst Witt showed how to put a ring structure on the set of Witt vectors, in such a way that the ring of Witt vectors over the finite field of order p is the ring of p-adic integers.

Motivation

We basically want to derive the ring p-adic integers p from the finite field with p elements, 𝔽p, by some general construction.

A p-adic integer is a sequence (n0,n1,...) with ni/p(n+1),such that ninjmodpi if i<j. They can be expanded as a power series in p: a0+a1p1+a2p2+..., where the a's are usually taken from the set {0,1,2,...,p1} (The equation is happening in p, with ai and pj all images from to p). Set-theoretically it is 𝔽p. But p is not isomorphic to 𝔽p. If we denote a+b=c, then the addition should instead be:

c0a0+b0modp
c0+c1pa0+a1p+b0+b1pmodp2
c0+c1p+c2p2a0+a1p+a2p2+b0+b1p+b2p2modp3

But we lack some properties of the coefficients to produce a general formula.

Luckily, there is an alternative subset of p which can work as the coefficient set. This is the Teichmüller representatives of elements of 𝔽p. Without 0 they form a subgroup of p*, identified with 𝔽p* through the Teichmüller character ω:𝔽p*p*. Note this is not additive, as the sum need not be a representative. Despite this, if ω(k)=ω(i)+ω(j)modp in p, then i+j=k in 𝔽p. This is conceptually justified by mω=id𝔽p if we denote m:pp/pp𝔽p.

They are explicitly calculated as roots of xp11=0 through Hensel lifting. For example, in 3, to calculate the representative of 2, you first find the unique solution of x21=0 in /9 with x2mod3; You get 8, then repeat it in /27, with conditions x21=0 and x2mod9; This time it is 26, and so on. The existence of lift in each step is guaranteed by (xp11,(p1)xp2)=1 in every /pn.

We can also write the representatives as a0+a1p1+a2p2+.... Note for every j{0,1,2,...,p1}, there is exactly one representative, namely ω(j), with a0=j, so we can also expand every p-adic integer as a power series in p, with coefficients from the Teichmüller representatives.

Explicitly, if b=a0+a1p1+a2p2+..., then bω(a0)=a'1p1+a'2p2+.... Then you subtract ω(a'1)p and proceed similarly. Note the coefficients you get most probably differ from the ai's, except the first one.

This time we have additional properties of the coefficients like aip=ai, so we can make some changes to get a neat formula. Since the Teichmüller character is not additive, we don't have c0=a0+b0 in p. But it happens in 𝔽p, as the first congruence implies. So actually c0p(a0+b0)pmodp2, thus c0a0b0(a0+b0)pa0b0(p1)a0p1b0+...+(p1)a0b0p1modp2. Since (pi) is divisible by p, this resolves the p-coefficient problem of c1 and gives c1a1+b1a0p1b0p12a0p2b02...a0b0p1modp. Note this completely determines c1 by the lift. Moreover, the modp indicates that the calculation can actually be done in 𝔽p, satisfying our basic aim.

Now for c2. It is already very cumbersome at this step. c1=c1p(a1+b1a0p1b0p12a0p2b02...a0b0p1)pmodp. As for c0, a single pth power is not enough: actually we take c0=c0p2(a0+b0)p2. (p2i) is not always divisible by p2, but that only happens when i=pd, in which case aibp2i=adbpd combined with similar monomials in c1p would make a multiple of p2.

At this step, we see that we are actually working with something like

c0a0+b0modp
c0p+c1pa0p+a1p+b0p+b1pmodp2
c0p2+c1pp+c2p2a0p2+a1pp+a2p2+b0p2+b1pp+b2p2modp3

This motivates the definition of Witt vectors.

Construction of Witt rings

Fix a prime number p. A Witt vector over a commutative ring R is a sequence :(X0,X1,X2,...) of elements of R. Define the Witt polynomials Wi by

  1. W0=X0
  2. W1=X0p+pX1
  3. W2=X0p2+pX1p+p2X2

and in general

Wn=ipiXipni.

(W0,W1,W2,...) is called the ghost components of the Witt vector (X0,X1,X2,...), and is usually denoted by (X(0),X(1),X(2),...).

Then Witt showed that there is a unique way to make the set of Witt vectors over any commutative ring R into a ring, called the ring of Witt vectors, such that

  • the sum and product are given by polynomials with integral coefficients that do not depend on R, and
  • Every Witt polynomial is a homomorphism from the ring of Witt vectors over R to R.

In other words, if

The first few polynomials giving the sum and product of Witt vectors can be written down explicitly. For example,

Examples

  • The Witt ring of any commutative ring R in which p is invertible is just isomorphic to RN (the product of a countable number of copies of R). In fact the Witt polynomials always give a homomorphism from the ring of Witt vectors to RN, and if p is invertible this homomorphism is an isomorphism.
  • The Witt ring of the finite field of order p is the ring of p-adic integers, as is demonstrated above.
  • The Witt ring of a finite field of order pn is the unramified extension of degree n of the ring of p-adic integers.

Universal Witt vectors

The Witt polynomials for different primes p are special cases of universal Witt polynomials, which can be used to form a universal Witt ring (not depending on a choice of prime p). Define the universal Witt polynomials Wn for n≥1 by

  1. W1=X1
  2. W2=X12+2X2
  3. W3=X13+3X3
  4. W4=X14+2X22+4X4

and in general

Wn=d|ndXdn/d.

Again, (W1,W2,W3,...) is called the ghost components of the Witt vector (X1,X2,X3,...), and is usually denoted by (X(1),X(2),X(3),...).

We can use these polynomials to define the ring of universal Witt vectors over any commutative ring R in much the same way as above (so the universal Witt polynomials are all homomorphisms to the ring R).

Generating Functions

Later Witt orally stated another approach using generating functions.[1]

Definition

Let X be a Witt vector and define

fX(t)=n1(1Xntn)=n0Antn

For n1 let 𝒮n denote the collection of subsets of {1,2,...,n} whose elements add up to n. Then An=S𝒮(1)|S|iSXi.

We can get the ghost components by taking the logarithmic derivative:

ddtlogfX(t)=n1ddt(1Xntn)=n1d1Xndtndd=m1d|mmdXmddmtm=m1X(m)tmm

Sum

Now we can see fZ(t)=fX(t)fY(t) if Z=X+Y. So that Cn=0inAnBni if An,Bn,Cn are respective coefficients in the power series for fX(t),fY(t),fZ(t). Then Zn=0inAnBniS𝒮,S{n}(1)|S|iSZi. Since An is a polynomial in X1,...,Xn and likely for Bn, we can show by induction that Zn is a polynomial in X1,...,Xn,Y1,...,Yn.

Product

If we set W=XY then

ddtlogfW(t)=m1X(m)Y(m)tmm

But

m1X(m)Y(m)mtm=m1d|mdXdm/de|meYem/emtm

Now 3-tuples m,d,e with m+,d|m,e|m are in bijection with 3-tuples d,e,n with d,e,n+, via n=m/[d,e] ([d,e] is the Least common multiple), our series becomes

d,e1de[d,e]n1(Xd[d,e]/dYe[d,e]/et[d,e])nn

So that

fW(t)=d,e1(1Xd[d,e]/dYe[d,e]/et[d,e])de/[d,e]=n0Dntn

where Dns are polynomials of X1,...,Xn,Y1,...,Yn. So by similar induction, suppose fW(t)=n1(1Wntn), then Wn can be solved as polynomials of X1,...,Xn,Y1,...,Yn.

Ring schemes

The map taking a commutative ring R to the ring of Witt vectors over R (for a fixed prime p) is a functor from commutative rings to commutative rings, and is also representable, so it can be thought of as a ring scheme, called the Witt scheme, over Spec(Z). The Witt scheme can be canonically identified with the spectrum of the ring of symmetric functions.

Similarly the rings of truncated Witt vectors, and the rings of universal Witt vectors, correspond to ring schemes, called the truncated Witt schemes and the universal Witt scheme .

Moreover, the functor taking the commutative ring R to the set Rn is represented by the affine space 𝔸n, and the ring structure on Rn makes 𝔸n into a ring scheme denoted 𝒪_n. From the construction of truncated Witt vectors it follows that their associated ring scheme 𝕎n is the scheme 𝔸n with the unique ring structure such that the morphism 𝕎n𝒪_n given by the Witt polynomials is a morphism of ring schemes.

Commutative unipotent algebraic groups

Over an algebraically closed field of characteristic 0, any unipotent abelian connected algebraic group is isomorphic to a product of copies of the additive group Ga. The analogue of this for fields of characteristic p is false: the truncated Witt schemes are counterexamples. (We make them into algebraic groups by forgetting the multiplication and just using the additive structure.) However these are essentially the only counterexamples: over an algebraically closed field of characteristic p, any unipotent abelian connected algebraic group is isogenous to a product of truncated Witt group schemes.

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, section II.6
  • 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
  • Greenberg, M. J. (1969), Lectures on Forms in Many Variables, New York and Amsterdam, Benjamin, Template:MathSciNet, ASIN: B0006BX17M
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