Originally Posted by

**Lord Darkin** If we have a 4x4 matrix A and it has rank = 3 (so nullity = 1) and the eigenvectors are 0, 0, 0, and some other nonzero number (so 0 is of multiplicity 3), then is A diagonalizable?

My understanding is that a matrix is diagonalizable if and only if it has linearly independent eigenvectors. It is possible for a matrix with a repeated eigenvalue to be diagonalizable if those repeated matrices form an eigenspace of dimension equal to their multiplicity.

Am I right in assuming that because the nullity is 1, that when the eigenvalue is 0, the N(A - LambdaI) = N(A + 0) = N(A) = dimension 1. That means that there are 3 eigenvalues, but there will be only 1 linearly independent vector that results because the basis is of dimension 1.

So this matrix cannot be diagonalizable.

This is supposed to be the right answer, but what if the other, nonzero eigenvalue (The 4th one) happened to have an eigenspace of dimension 3, so it could create 3 linearly independent vectors. Or is that not possible?