Poly-vector
-
vector, over a vector space V
An element of the p - th exterior degree \wedge ^ {p} V of the space V over a field K ( see Exterior algebra). A p - vector can be understood as a p - times skew-symmetrized contravariant tensor on V . Any linearly independent system of vectors x _ {1} \dots x _ {p} from V defines a non-zero p - vector x _ {1} \wedge \dots \wedge x _ {p} ; such a poly-vector is called factorable, decomposable, pure, or prime (often simply a poly-vector). Here two linearly independent systems x _ {1} \dots x _ {p} and y _ {1} \dots y _ {p} generate the same subspace in V if and only if y _ {1} \wedge \dots \wedge y _ {p} = cx _ {1} \wedge \dots \wedge x _ {p} , where c \in K . For any non-zero poly-vector t \in \wedge ^ {p} V , its annihilator \mathop{\rm Ann} t = \{ {v \in V } : {t \wedge v = 0 } \} is a subspace of dimension \leq p , and the poly-vector t is pure if and only if \mathop{\rm dim} \mathop{\rm Ann} t = p . The pure p - vectors of an n - dimensional space V form an algebraic variety in \wedge ^ {p} V ; the corresponding projective algebraic variety is a Grassmann manifold. Any non-zero n - vector or ( n- 1) - vector in an n - dimensional space V is pure, but a bivector t is pure if and only if t \wedge t = 0 .
If v _ {1} \dots v _ {n} is a basis of V and x _ {i} = \sum_{j=1} ^ {n} x _ {i} ^ {j} v _ {j} , then the coordinates of the poly-vector t = x _ {1} \wedge \dots \wedge x _ {p} in the basis \{ {v _ {i _ {1} } \wedge \dots \wedge v _ {i _ {p} } } : {i _ {1} < \dots < i _ {p} } \} of the space \wedge ^ {p} V are the minors t ^ {i _ {1} \dots i _ {p} } = \mathop{\rm det} \| x _ {i} ^ {i _ {k} } \| , i _ {1} < \dots < i _ {p} , of the matrix \| x _ {i} ^ {j} \| . In particular, for p = n ,
x _ {1} \wedge \dots \wedge x _ {n} = \ \mathop{\rm det} \| x _ {i} ^ {j} \| v _ {1} \wedge \dots \wedge v _ {n} .
If one specifies a non-zero n - vector \omega \in \wedge ^ {n} V , a duality between p - vectors and ( n- p) - vectors is obtained, i.e. a natural isomorphism
\pi : \wedge ^ {p} ( V) \rightarrow \ ( \wedge ^ {n-p} V) ^ {*} \cong \wedge ^ {n-p} ( V ^ {*} )
such that t \wedge u = \pi ( t)( u) \omega for all t \in \wedge ^ {p} V and u \in \wedge ^ {n-p} V .
Let k = \mathbf R and let an inner product be defined in V , then in \wedge ^ {p} V an inner product is induced with the following property: For any orthonormal basis v _ {1} \dots v _ {n} in V the basis \{ {v _ {i _ {1} } \wedge \dots \wedge v _ {i _ {p} } } : {i _ {1} < \dots < i _ {p} } \} in \wedge ^ {p} V is also orthonormal. The scalar square
( t, t) = \sum _ {i _ {1} < \dots < i _ {p} } ( t ^ {i _ {1} \dots i _ {p} } ) ^ {2}
of a pure poly-vector t = x _ {1} \wedge \dots \wedge x _ {p} coincides with the square of the volume of the parallelopipedon in V constructed on the vectors x _ {1} \dots x _ {p} . If one specifies an orientation in the n - dimensional Euclidean space V ( which is equivalent to choosing an n - vector \omega for which ( \omega , \omega ) = 1 ), then the above duality leads to a natural isomorphism \gamma : \wedge ^ {p} V \rightarrow \wedge ^ {n-p} V . In particular, the ( n- 1) - vector t = x _ {1} \wedge \dots \wedge x _ {n-1} corresponds to a vector \gamma ( t) \in V , called the vector product of the vectors x _ {1} \dots x _ {n-1} .
References
[1] | N. Bourbaki, "Elements of mathematics. Algebra: Algebraic structures. Linear algebra" , 1 , Addison-Wesley (1974) pp. Chapt.1;2 (Translated from French) |
[2] | A.I. Kostrikin, Yu.I. Manin, "Linear algebra and geometry" , Gordon & Breach (1989) (Translated from Russian) |
[3] | M.M. Postnikov, "Linear algebra and differential geometry" , Moscow (1979) (In Russian) |
Poly-vector. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Poly-vector&oldid=54952