The Fourier cosine transform is the Real Part of the full complex Fourier Transform,
See also Fourier Sine Transform, Fourier Transform
References
Press, W. H.; Flannery, B. P.; Teukolsky, S. A.; and Vetterling, W. T. ``FFT of Real Functions, Sine and Cosine Transforms.'' §12.3 in
Numerical Recipes in FORTRAN: The Art of Scientific Computing, 2nd ed. Cambridge, England:
Cambridge University Press, pp. 504-515, 1992.