Square Matrices:
If a matrix has similar number of rows and columns, for illustration, if m == n, the matrix is square matrix. The definitions which follow in this part apply only to the square matrices.
The main diagonal of a square matrix is the set of terms a_{ii} for which the row and column indices are similar, for illustration, from the upper left element to the lower right. For illustration, for the matrix below it is the set of numbers 1, 6, 11, & 16.
This is sometimes just called the diagonal. The MATLAB has a function diag which will return the diagonal of a matrix as a column vector.
>> mymat = reshape(1:16,4,4)'
mymat =
1 2 3 4
5 6 7 8
9 10 11 12
13 14 15 16
>> diag(mymat)'
ans =
1 6 11 16