** Create or extract diagonals. **

** If V is a row or column vector with N components, **

** DIAG(V,K) is a square matrix of order N+ ABS (K) with the **

** elements of V on the K-th diagonal. K = 0 is the main **

** diagonal, K > 0 is above the main diagonal and K < 0 is **

** below the main diagonal. DIAG(V) simply puts V on the **

** main diagonal. For example, **

** DIAG(-M:M) + DIAG( ONES (2*M,1),1) + DIAG(ONES (2*M,1),-1) **

** produces a tridiagonal matrix of order 2*M+1. **

** **

** **

** If X is a matrix, DIAG(X,K) is a column vector formed from **

** the elements of the K-th diagonal of X. DIAG(X) is the main **

** diagonal of X. DIAG(DIAG(X)) is a diagonal matrix. **

** **