I want to multiply two complex matrices AT*A (matrix A is 121x14) to fit data using least squares . I tried two ways: MATMUL and MKL ZGEMM function. But I obtained different results and even more different results than in MATLAB (I suppose MATLAB is correct because results of LSQ fit are correct).
The results are:
element (1,3) is different in all 3 versions and difference is on the 3rd decimal place which is unacceptable for me.
I am attaching code (windows, Visual Fortran Compiler XE 18.104.22.168) used to produce these results.
What to do to improve precision of results?