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Nullspaces of a Matrix

by Big-oh
Tags: matrix, nullspaces
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Feb11-12, 05:50 PM
P: 5
1. The problem statement, all variables and given/known data

Given an mxn matrix M, prove that Null((M^T)(M)) = Null(M)

Where M^T is the transpose of the matrix M.

3. The attempt at a solution

I was able to get the first part (Null(M) is a subset of Null((M^T)(M))), but I'm just having trouble proving the other way around. I pick any vector in Null((M^T)(M)), but unsure of what to do after that.
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The standard way to prove that to sets, A and B, say, are equal is to prove A is a subset of B then prove that B is a subset of A. And you prove A is a subset of B by starting "if x is in A" and then use the properties of A and B to conclude "x is in B".

Here, the two sets are Null(M) and Null(M^T(M)). If x is in Null(M) then M(x)= 0. It then follows immediately that M^T(Mx)= M^T(0)= 0. That's the easy way. If x is in Null(M^T(M)) then M^T(M(x))= 0. Obviously, if M(x)= 0 we are done. What can you say about non-zero x such that M^t(x)= 0?

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