Txema said:
TL;DR Summary: Can a vector subspace have the same dimension as the space it is part of?
Can a vector subspace have the same dimension as the space it is part of?
If so, can such a subspace have a Cartesian equation?
if so, can you give an example.
Thanks in advance;
The dimension of a vector space is the number of vectors in its basis.
Each basis of a vector space has the same number of vectors and ##dim(Ssub) \leq dim(S)##.
PeroK said:
Technically, a vector space is a subspace of itself. This is sometimes called a trivial subspace or not a proper subspace. See, for example:
The Cartesian equation (If it is a scalar product that determines the subspace) is also trivial.
The scalar product with zero vector
##\langle 0,x \rangle=0 \cdot x_1+0 \cdot x_2+...+0 \cdot x_n=0##
That is it in finite dimensional spaces .
When the vector space has infinite dimensions, you can use the property that ##\infty+1## is still ##\infty##.
You can choose the set of all infinite sequences as a vector space ##x=(x_1, x_2, x_3, ... ) \in S##
and all sequences which sum is zero as a subspace ##Ssub## . Its dimension is ##\infty##.
The Cartesian equation in that situation is
##\langle 1,x \rangle=1 \cdot x_1+1 \cdot x_2+...+1 \cdot x_n +...=0##
where 1 is infinite series ##(1, 1, 1, ... )##
All constant sequences ( which all members are the same number ) is a subspace . Its dimension is 1.
e.g. ##(1, 1, 1, ...)##, ##(-4, -4, -4, ... ) ##, ...
A proper, non-trivial, subspace with the same dimension as the space can only be obtained if the vector space is infinite-dimensional.