- #1
sschmiggles
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Homework Statement
A is a 2 x 2 matrix. Prove that the set W = {X: XA = AX} is a subspace of M2,2
Homework Equations
The Attempt at a Solution
I have already proven non-emptiness and vector addition.
Non-emptiness:
Code:
W must be non-empty because the identity matrix I is an element of W.
IA = AI
A = A
Vector addition:
Code:
Let X, Y be elements of W such that XA = AX, YA = AY. Add the equations together.
(X+Y)A = A(X+Y)
XA+YA = AX+AY
Hence, X+Y is an element of W.
I have no idea what to do with scalar multiplication. This is my current attempt:
Code:
Let X be an element of W, and c be a scalar.
cXA = AcX
(cX)A = A(cX)
I also tried this:
Code:
Let X be an element of W, and c be a scalar.
XA = AX
c(XA) = c(AX)
I don't feel like either of these attempts proves anything.
I don't understand how to prove that cX is actually in W. This doesn't make any sense to me. How do I prove that?