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| {{see also|orientation (geometry)}}
| | Emilia Shryock is my name but you can call me something you like. What I love doing is to collect badges but I've been taking on new issues recently. Minnesota has always been his house but his spouse desires them to transfer. Bookkeeping is my profession.<br><br>Feel free to surf to my web site ... [http://zhadui.com/space.php?uid=259109&do=blog&id=852234 zhadui.com] |
| [[File:Cartesian coordinate system handedness.svg|thumb|The left-handed orientation is shown on the left, and the right-handed on the right.]]
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| In [[mathematics]], '''orientation''' is a geometric notion that in two dimensions allows one to say when a cycle goes around clockwise or counterclockwise, and in three dimensions when a figure is left-handed or right-handed. In [[linear algebra]], the notion of orientation makes sense in arbitrary dimensions. In this setting, the orientation of an [[ordered basis]] is a kind of asymmetry that makes a [[reflection (mathematics)|reflection]] impossible to replicate by means of a simple [[rotation (mathematics)|rotation]]. Thus, in three dimensions, it is impossible to make the left hand of a human figure into the right hand of the figure by applying a rotation alone, but it is possible to do so by reflecting the figure in a mirror. As a result, in the three-dimensional [[Euclidean space]], the two possible basis orientations are called [[right hand rule|right-handed]] and left-handed (or right-chiral and left-chiral).
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| The orientation on a [[real number|real]] [[vector space]] is the arbitrary choice of which ordered bases are "positively" oriented and which are "negatively" oriented. In the three-dimensional [[Euclidean space]], right-handed bases are typically declared to be positively oriented, but the choice is arbitrary, as they may also be assigned a negative orientation. A vector space with an orientation is called an '''oriented''' vector space, while one without a choice of orientation is called '''{{visible anchor|unoriented}}'''.
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| ==Definition==
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| Let ''V'' be a [[finite-dimensional]] real vector space and let ''b''<sub>1</sub> and ''b''<sub>2</sub> be two ordered bases for ''V''. It is a standard result in [[linear algebra]] that there exists a unique [[linear transformation]] ''A'' : ''V'' → ''V'' that takes ''b''<sub>1</sub> to ''b''<sub>2</sub>. The bases ''b''<sub>1</sub> and ''b''<sub>2</sub> are said to have the ''same orientation'' (or be consistently oriented) if ''A'' has positive [[determinant]]; otherwise they have ''opposite orientations''. The property of having the same orientation defines an [[equivalence relation]] on the set of all ordered bases for ''V''. If ''V'' is non-zero, there are precisely two [[equivalence class]]es determined by this relation. An '''orientation''' on ''V'' is an assignment of +1 to one equivalence class and −1 to the other.<ref>Rowland, Todd. "Vector Space Orientation." From MathWorld--A Wolfram Web Resource, created by Eric W. Weisstein. http://mathworld.wolfram.com/VectorSpaceOrientation.html</ref>
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| Every ordered basis lives in one equivalence class or another. Thus any choice of a privileged ordered basis for ''V'' determines an orientation: the orientation class of the privileged basis is declared to be positive. For example, the [[standard basis]] on '''R'''<sup>''n''</sup> provides a '''standard orientation''' on '''R'''<sup>''n''</sup> (in turn, the orientation of the standard basis depends on the orientation of the [[Cartesian coordinate system]] on which it is built). Any choice of a linear [[isomorphism]] between ''V'' and '''R'''<sup>''n''</sup> will then provide an orientation on ''V''.
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| The ordering of elements in a basis is crucial. Two bases with a different ordering will differ by some [[permutation]]. They will have the same/opposite orientations according to whether the [[signature (permutation)|signature]] of this permutation is ±1. This is because the determinant of a [[permutation matrix]] is equal to the signature of the associated permutation.
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| Similarly, let ''A'' be a nonsingular linear mapping of vector space '''R'''<sup>''n''</sup> to '''R'''<sup>''n''</sup>. This mapping is '''orientation-preserving''' if its determinant is positive.<ref>
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| Weisstein, Eric W. "Orientation-Preserving." From MathWorld--A Wolfram Web Resource. http://mathworld.wolfram.com/Orientation-Preserving.html</ref> For instance, in '''R'''<sup>3</sup> a rotation around the ''Z'' Cartesian axis by an angle ''α'' is orientation-preserving:
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| ::<math>
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| \bold {A}_1 = \begin{pmatrix}
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| \cos \alpha & -\sin \alpha & 0 \\
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| \sin \alpha & \cos \alpha & 0 \\
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| 0 & 0 & 1
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| \end{pmatrix}
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| </math>
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| while a reflection by the ''XY'' Cartesian plane is not orientation-preserving:
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| ::<math>
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| \bold {A}_2 = \begin{pmatrix}
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| 1 & 0 & 0 \\
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| 0 & 1 & 0 \\
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| 0 & 0 & -1
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| \end{pmatrix}
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| </math>
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| ===Zero-dimensional case===
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| The concept of orientation defined above did not quite apply to zero-dimensional vector spaces (as the only empty matrix is the identity (with determinant 1), so there will be only one equivalence class). However, it is useful to be able to assign different orientations to a point (e.g. orienting the boundary of a 1-dimensional manifold). A more general definition of orientation that works regardless of dimension is the following: An orientation on ''V'' is a map from the set of ordered bases of ''V'' to the set <math>\{\pm 1\}</math> that is invariant under base changes with positive determinant and changes sign under base changes with negative determinant (it is equivarient with respect to the homomorphism <math>GL_n \to \pm 1</math>). The set of ordered bases of the zero-dimensional vector space has one element (the empty set), and so there are two maps from this set to <math>\{\pm 1\}</math>.
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| A subtle point is that a zero-dimensional vector space is naturally (canonically) oriented, so we can talk about an orientation being positive (agreeing with the canonical orientation) or negative (disagreeing). An application is interpreting the [[Fundamental theorem of calculus]] as a special case of [[Stokes' theorem]].
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| Two ways of seeing this are:
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| * A zero-dimensional vector space is a point, and there is a unique map from a point to a point, so every zero-dimensional vector space is naturally identified with '''R'''<sup>0</sup>, and thus is oriented.
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| * The 0th exterior power of a vector space is the ground field <math>K</math>, which here is '''R'''<sup>1</sup>, which has an orientation (given by the standard basis).
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| ==Alternate viewpoints==
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| ===Multilinear algebra===
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| For any ''n''-dimensional real vector space ''V'' we can form the ''k''<sup>th</sup>-[[exterior power]] of ''V'', denoted Λ<sup>''k''</sup>''V''. This is a real vector space of dimension [[binomial coefficient|<math>\tbinom{n}{k}</math>]]. The vector space Λ<sup>''n''</sup>''V'' (called the ''top exterior power'') therefore has dimension 1. That is, Λ<sup>''n''</sup>''V'' is just a real line. There is no ''a priori'' choice of which direction on this line is positive. An orientation is just such a choice. Any nonzero [[linear form]] ω on Λ<sup>''n''</sup>''V'' determines an orientation of ''V'' by declaring that ''x'' is in the positive direction when ω(''x'') > 0. To connect with the basis point of view we say that the positively oriented bases are those on which ω evaluates to a positive number (since ω is an ''n''-form we can evaluate it on an ordered set of ''n'' vectors, giving an element of '''R'''). The form ω is called an '''orientation form'''. If {''e''<sub>''i''</sub>} is a privileged basis for ''V'' and {''e''<sub>''i''</sub><sup>*</sup>} is the [[dual basis]], then the orientation form giving the standard orientation is ''e''<sub>1</sub><sup>*</sup>∧''e''<sub>2</sub><sup>*</sup>∧…∧''e''<sub>''n''</sub><sup>*</sup>.
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| The connection of this with the determinant point of view is:
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| the determinant of an endomorphism <math>T\colon V \to V</math> can be interpreted as the induced action on the top exterior power.
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| ===Lie group theory===
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| Let ''B'' be the set of all ordered bases for ''V''. Then the [[general linear group]] GL(''V'') [[group action|acts]] freely and transitively on ''B''. (In fancy language, ''B'' is a GL(''V'')-[[torsor]]). This means that as a [[manifold]], ''B'' is (noncanonically) [[homeomorphic]] to GL(''V''). Note that the group GL(''V'') is not [[connected space|connected]], but rather has two [[connected space|connected component]]s according to whether the determinant of the transformation is positive or negative (except for GL<sub>0</sub>, which is the trivial group and thus has a single connected component; this corresponds to the canonical orientation on a zero-dimensional vector space). The [[identity component]] of GL(''V'') is denoted GL<sup>+</sup>(''V'') and consists of those transformations with positive determinant. The action of GL<sup>+</sup>(''V'') on ''B'' is ''not'' transitive: there are two orbits which correspond to the connected components of ''B''. These orbits are precisely the equivalence classes referred to above. Since ''B'' does not have a distinguished element (i.e. a privileged basis) there is no natural choice of which component is positive. Contrast this with GL(''V'') which does have a privileged component: the component of the identity. A specific choice of homeomorphism between ''B'' and GL(''V'') is equivalent to a choice of a privileged basis and therefore determines an orientation.
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| More formally: <math>\pi_0(GL(V)) = (GL(V)/GL^+(V) = \{\pm 1\}</math>,
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| and the [[Stiefel manifold]] of n-frames in <math>V</math> is a <math>GL(V)</math>-[[torsor]], so <math>V_n(V)/GL^+(V)</math> is a [[torsor]] over <math>\{\pm 1\}</math>, i.e., it's 2 points, and a choice of one of them is an orientation.
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| ===Geometric algebra===
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| [[File:Wedge product.JPG|thumb|left|150px|Parallel plane segments with the same attitude, magnitude and orientation, all corresponding to the same bivector {{nowrap|'''a''' ∧ '''b'''}}.<ref name=Dorst>
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| {{cite book |author=Leo Dorst, Daniel Fontijne, Stephen Mann |title=Geometric Algebra for Computer Science: An Object-Oriented Approach to Geometry |url=http://books.google.com/books?id=-1-zRTeCXwgC&pg=PA32#v=onepage&q=&f=false |page=32 |isbn=0-12-374942-5 |publisher=Morgan Kaufmann |year=2009 |edition=2nd| quote=The algebraic bivector is not specific on shape; geometrically it is an amount of oriented area in a specific plane, that's all.}}</ref>]]
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| The various objects of [[geometric algebra]] are charged with three attributes or ''features'': attitude, orientation, and magnitude.<ref name=Jancewicz1>
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| {{cite book |author=B Jancewicz |url=http://books.google.com/books?id=0Nji78YQKfQC&pg=PA403&dq=attitude+%22Table+28.1%22&lr=&as_drrb_is=q&as_minm_is=0&as_miny_is=&as_maxm_is=0&as_maxy_is=&as_brr=0&cd=5#v=onepage&q=attitude%20%22Table%2028.1%22&f=false |page=397 |editor=William Eric Baylis |chapter=Tables 28.1 & 28.2 in section 28.3: '' Forms and pseudoforms'' |isbn=0-8176-3868-7 |year=1996 |publisher=Springer |title=Clifford (geometric) algebras with applications to physics, mathematics, and engineering}}
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| </ref> For example, a [[Euclidean vector|vector]] has an attitude given by a straight line parallel to it, an orientation given by its sense (often indicated by an arrowhead) and a magnitude given by its length. Similarly, a [[bivector]] in three dimensions has an attitude given by the family of [[Plane (geometry)|plane]]s associated with it (possibly specified by the [[Tangential and normal components|normal line]] common to these planes <ref name=Granville>
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| {{cite book |author=William Anthony Granville |title =Elements of the differential and integral calculus |page=275 |url=http://books.google.com/books?id=0jcAAAAAYAAJ&pg=PA275 |chapter=§178 Normal line to a surface |publisher=Ginn & Company |year=1904}}
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| </ref>), an orientation (sometimes denoted by a curved arrow in the plane) indicating a choice of sense of traversal of its boundary (its ''circulation''), and a magnitude given by the area of the parallelogram defined by its two vectors.<ref name=Hestenes>
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| {{cite book |title=New foundations for classical mechanics: Fundamental Theories of Physics |author=David Hestenes |url=http://books.google.com/books?id=AlvTCEzSI5wC&pg=PA21 |page=21 |isbn=0-7923-5302-1 |edition=2nd |year=1999 |publisher=Springer| authorlink = David Hestenes}}
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| </ref>
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| ==Orientation on manifolds==
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| {{Main|Orientability}}
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| [[File:Surface orientation.pdf|thumb|The orientation of a volume may be determined by the orientation on its boundary, indicated by the circulating arrows.]]
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| One can also discuss orientation on [[manifold]]s. Each point ''p'' on an ''n''-dimensional differentiable manifold has a [[tangent space]] ''T''<sub>''p''</sup>''M'' which is an ''n''-dimensional real vector space. One can assign to each of these vector spaces an orientation. However, one would like to know whether it is possible to choose the orientations so that they "vary smoothly" from point to point. Due to certain [[topology|topological]] restrictions, there are situations when this is impossible. A manifold which admits a smooth choice of orientations for its tangents spaces is said to be ''orientable''. See the article on [[orientability]] for more on orientations of manifolds.
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| ==See also==
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| *[[Sign convention]]
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| *[[Rotation formalisms in three dimensions]]
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| *[[Chirality (mathematics)]]
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| *[[Right-hand rule]]
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| *[[Even and odd permutations]]
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| *[[Cartesian coordinate system]]
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| *[[Attitude (geometry)]]
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| *[[Pseudovector]] — Pseudovectors are a consequence of oriented spaces.
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| *[[Orientability]] — Discussion about the possibility of having orientations in a space.
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| ==References==
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| {{Reflist}}
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| ==External links==
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| * {{springer|title=Orientation|id=p/o070200}}
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| {{DEFAULTSORT:Orientation (vector space)}}
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| [[Category:Linear algebra]]
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| [[Category:Analytic geometry]]
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| [[Category:Orientation]]
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Emilia Shryock is my name but you can call me something you like. What I love doing is to collect badges but I've been taking on new issues recently. Minnesota has always been his house but his spouse desires them to transfer. Bookkeeping is my profession.
Feel free to surf to my web site ... zhadui.com