1. The problem statement, all variables and given/known data show the set of nxn diagonal matrices is a group under matrix addition 2. Relevant equations 3. The attempt at a solution it doesn't say what set the entries are from, so i'm assuming it's reals. so i need to show that there is closure, it's associative, there's an identity element, and there's an inverse. i know that there's an identity element, the matrix with just zeroes, and i know the inverse is just -A for matrix A. it's addition, so i know that it's associative, and in my head i can tell that there is closure. my main problem is notation. how to do i actually express this? i.e., what's the general notation for a nxn matrix?