(3) If X is an nonnegative-definite solution to AXA*=B, we give the formula for the rank of X and the conditions under which X is a maximal or minimal rank solution.

• (3)若X是矩阵方程AXA*=B的非负定解；给出了求X的秩的公式以及X为最小秩或最大秩解的条件.