Nullspaces of a Matrix


by Big-oh
Tags: matrix, nullspaces
Big-oh
Big-oh is offline
#1
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.
Phys.Org News Partner Science news on Phys.org
SensaBubble: It's a bubble, but not as we know it (w/ video)
The hemihelix: Scientists discover a new shape using rubber bands (w/ video)
Microbes provide insights into evolution of human language
HallsofIvy
HallsofIvy is offline
#2
Feb12-12, 12:57 PM
Math
Emeritus
Sci Advisor
Thanks
PF Gold
P: 38,904
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?


Register to reply

Related Discussions
NxN-complex matrix, identified 2Nx2N-real matrix, determinant Linear & Abstract Algebra 2
real matrix -- eigenvector matrix R -- diagonal eigenvalue matrix L Programming & Computer Science 0
Intersection of nullspaces Linear & Abstract Algebra 2
Coding Covariance Matrix (differential Riccati matrix eqn) of the Kalman-Bucy Filter Engineering, Comp Sci, & Technology Homework 0
Prove that Hermitian/Skew Herm/Unitary Matrix is a Normal Matrix Calculus & Beyond Homework 2