Tracing of Square matrices:
The trace of a square matrix is the addition of all the elements on the diagonal. For illustration, for the preceding matrix it is 1 + 6 + 11 + 16, or 34.
The square matrix is symmetric if aij = aji for all i, j. In another words, all the values opposite to the diagonal from each other should be equal to each other. In this illustration, there are three pairs of values opposite to the diagonals, all of which are equal that is the 2's, the 9's, and the 4's.
The square matrix is a diagonal matrix if all values which are not on the diagonal are 0. The numbers on the diagonal, though, do not have to be all nonzero though often they are. Mathematically, this is written as aij = 0 for i ~= j.
An example of a diagonal matrix is shown here.