Abstract
Let \(A\) and \(B\) be matrices of order \(n\) that are direct sums of nilpotent Jordan blocks. Suppose that \(A\) and \(B\) are not just different arrangements of the same blocks but, rather, they differ in the sizes of the blocks. It is shown that, in this case, \(A\) and \(B\) cannot be congruent. This result can be regarded as a new proof of the uniqueness of the singular part in the Horn–Sergeichuk canonical form of a singular matrix.