Hello, I have a question. If A and B are NND matrices, how to prove C(A) belongs to C(A+B)? I can prove that C(A)<C(A,B) by using A=(A,B)transpose[(I,0)], and I also can prove C(A+B)<C(A,B) using the similar approach. But I cannot move further because my thoughs maybe not related to the answer at all. Can someone help? Many thanks.