# Thread: Prove: for any mxn matrix A, Rank(A)=0 iff A is zero Matrix

1. ## Prove: for any mxn matrix A, Rank(A)=0 iff A is zero Matrix

Prove: for any mxn matrix A, Rank(A)=0 iff A is zero Matrix

I know this is easy but I'm just no good at proofs. Can someone help me out?

1) Rank M is the maximal number of linearly independent columns (or rows) of M. So if M is not the zero matrix, it has at least one non-zero column (or row), and a set containing one non-zero vector is linearly independent. So rank M is at least 1.

2) Rank M is the dimension of the image of M, i.e. the vector space consisting of Mv for every vector V. If even one entry of M is nonzero, then there's a vector v (try one with all zeros except a 1 in one place) such that Mv is nonzero, and therefore the dimension of the image of M is not zero.

That's the "only if" direction. The "if" direction should be straightforward.

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# rank of a zero matrix is

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