Topic: dsygv not working as expected
I am using fortran on windows machine. My program generates two matrices A and B – both square, symmetric and positive definite. These matrices were generated based on a problem input from my side into the fortran program. I tried 6 independent runs with slight variation in inputs – for 5 cases, it ran well, the eigen values were matching my independent calcs using a software and also with Mathcad. For the 6th case, the dsygv listing gives 2 negative numbers as eigen values which was not expected. I took these matrices in Mathcad, and I got the answers matching software calculations. So this means, the matrices A and B are ok. could you please assist me to find why they are not working with dsygv. While comparing the dsygv listing with Mathcad, I found that eigen value(3) in dsygv was matching eigenval(1) of mathcad, eigenval(4) was matching eigenval(2) of mathcad and so on.. looks like the 2 negative values got inserted in the listing - could some one explain why this is happening. the A and B matrices are generated inside fortran and solved using dsygv. In the 6th case, there is some discrepancy(only while using dsygv, it works as usual while using mathcad/or independent software) - any guidance is appreciated.