Zero-dilation index of a finite matrix

For an n-by-n complex matrix A, we define its zero-dilation index d(A) as the largest size of a zero matrix which can be dilated to A. This is the same as the maximum k (⩾1) for which 0 is in the rank-k numerical range of A. Using a result of Li and Sze, we show that if d(A)>⌊2n/3⌋, then, under unitary similarity, A has the zero matrix of size 3d(A)−2n as a direct summand. It complements the known fact that if d(A)>⌊n/2⌋, then 0 is an eigenvalue of A. We then use it to give a complete characterization of n-by-n matrices A with d(A)=n−1, namely, A satisfies this condition if and only if it is unitarily similar to B⊕0n−3, where B is a 3-by-3 matrix whose numerical range W(B) is an elliptic disc and whose eigenvalue other than the two foci of ∂W(B) is 0. We also determine the value of d(A) for any normal matrix A and any weighted permutation matrix A with zero diagonals.