Can you provide an example of your singular matrix and what IDL functions you used and what error you are running into? Also what version of IDL are you using? Perhaps whatever you are running into has been fixed....

I am using IDL 8.0.1, I used LA_EIGENPROBLEM/HQR & EIGENVEC, cause the matrix I am using is an upper triangular adjacency matrix, it is asymmetrical and not full rank. But I then realized it was not because of it is singular, but its eigenvalues are all zero. But I then went back to test both LA_EIGENQL AND EIGENQL with matrix like: [0 2 3 4 5 0 0 5 6 7 0 0 0 5 7 0 0 0 5 6 0 0 0 0 0] LA_EIGENQL works, it gives : 0.000000 0.000000 0.000000 0.000000 5.00000 but for EIGENQL , ENVI> % EIGENQL: Input array must be symmetric. In IDL help, both LA_EIGENQL AND EIGENQL requires an n-by-n real symmetric or complex Hermitian array A. I wonder why they give me different results. Thanks.

Hi Florrie, The IDL documentation says that the matrix, for both functions, has to be a symetric array. So, I'm not sure why the LA_EIGENQL is actually given an answer at all: the input matrix needs to be "an n-by-n real symmetric or complex Hermitian array A". Is the result you see the correct answer?, ie. is the set of eigenvalues, i.e [0.000000 0.000000 0.000000 0.000000 5.00000 ], correct? For the other function, EIGENQL, the matrix just need to be symetric. See that the two examples are using symetric matrixes. Cheers, Fernando