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In mathematics, the dimension of a vector space V is the cardinality (i.e. the number of vectors) of a basis of V. It is sometimes called Hamel dimension to distinguish it from other types of dimension. All bases of a vector space have equal cardinality (see dimension theorem for vector spaces) and so the Hamel dimension of a vector space is uniquely defined. The dimension of the vectorspace V over the field F is written as dimF(V).
We say V is finite-dimensional if the dimension of V is finite.
Examples
The vector space R3 has {(1,0,0), (0,1,0), (0,0,1)} as a basis, and therefore we have dimR(R3) = 3. More generally, dimR(Rn) = n. And more generally still, dimF(Fn) = n.
The complex numbers C are a real vector space; we have dimR(C) = 2 and dimC(C) = 1. So the Hamel dimension depends on the base field.
The only vector space with dimension 0 is {0}, the vector space consisting only of its zero element.
Facts
If W is a linear subspace of V, then dim(W) ≤ dim(V).
To show that two finite-dimensional vector spaces are equal, one often uses the following criterion: if V is a finite-dimensional vector space and W is a linear subspace of V with dim(W) = dim(V), then W = V.
Any two vectorspaces over F having the same dimension are isomorphic. Any bijective map between their bases can be uniquely extended to a bijective linear map between the vector spaces. If B is some set, a vectorspace with dimension |B| over F can be constructed as follows: take the set F(B) of all functions f : B → F such that f(b) = 0 for all but finitely many b in B. These functions can be added and multiplied with elements of F, and we obtain the desired F-vectorspace.
An important result about dimensions related to a linear transformation is given by the rank-nullity theorem.
If F/K is a field extension, then F is in particular a vector space over K. Furthermore, every F-vector space V is also a K-vector space. The dimensions are related by the formula
- dimK(V) = dimK(F) dimF(V).
In particular, every complex vector space of dimension n is a real vector space of dimension 2n.
Some simple formulae relate the Hamel dimension of a vector space with the cardinality of the base field and the cardinality of the space itself.
If V is a vector space over a field F then, denoting the Hamel dimension of V by dimV, we have:
- If dimV is finite, then |V| = |F|dimV.
- If dimV is infinite, then |V| = max(|F|, dimV).
Generalizations
The length of a module and the rank of an abelian group both have several properties similar to the Hamel dimension of vector spaces.
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