**Q:**
I'm looking for some
code to do:

Fast Fourier Transforms (FFT's)

matrix arithmetic (multiplication, inversion, etc.)

complex arithmetic

**A:**
Ajay Shah
has prepared a nice
index of free numerical
software
which has been archived pretty widely;
one URL
is
`ftp://ftp.math.psu.edu/pub/FAQ/numcomp-free-c`
.
See also
questions 18.9b, 18.13, 18.15c, and 18.16.

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