fourier series coefficients.

[truble@mill]

I'm using fourier_an and fourier_bn. Results are unreliable. Result is often 'undef'. See attached.
an_setup_1 shows setup for cosine coefficients.
an_setup_2 shows result. Undef.
an_setup_help_page shows Prime help suggestions.
fourier_bn working example shows typical results expected.

BTW, fourier_an results should be +/- 8/n*pi for n odd and 0 for n even.
Can anyone explain my error? Thank you.