A matrix with 0 determinant whose determinant becomes nonzero when any element on or below the diagonal is changed from 0 to 1. An example is M = [1 | -1 | 0 | 0 0 | 0 | -1 | 0 1 | 1 | 1 | -1 0 | 0 | 1 | 0]. There are 2^(n - 1) minimal special matrices of size n×n.