The Fourier sine transform of f(t), sometimes denoted by either or , is
If t means time, then ξ is frequency in cycles per unit time, but in the abstract, they can be any pair of variables which are dual to each other.
This transform is necessarily an
odd function of frequency, i.e. for all ξ:
The numerical factors in the
Fourier transforms are defined uniquely only by their product. Here, in order that the Fourier inversion formula not have any numerical factor, the factor of 2 appears because the sine function has L2 norm of
The Fourier cosine transform of f(t), sometimes denoted by either or , is
It is necessarily an
even function of frequency, i.e. for all ξ:
Since positive frequencies can fully express the transform, the non-trivial concept of
negative frequency needed in the regular Fourier transform can be avoided.
Simplification to avoid negative t
Some authors[2] only define the cosine transform for
even functions of t, in which case its sine transform is zero. Since cosine is also even, a simpler formula can be used,
Similarly, if f is an
odd function, then the cosine transform is zero and the sine transform can be simplified to
Other conventions
Just like the Fourier transform takes the form of different equations with different constant factors (see
Fourier transform § Other conventions), other authors also define the cosine transform as[3]
using as the transformation variable. And while t is typically used to represent the time domain, x is often used alternatively, particularly when representing frequencies in a spatial domain.
Fourier inversion
The original function f can be recovered from its transform under the usual hypotheses, that f and both of its transforms should be absolutely integrable. For more details on the different hypotheses, see
Fourier inversion theorem.
which has the advantage that all quantities are real. Using the addition formula for
cosine, this can be rewritten as
If the original function f is an
even function, then the sine transform is zero; if f is an
odd function, then the cosine transform is zero. In either case, the inversion formula simplifies.
Using standard methods of numerical evaluation for Fourier integrals, such as Gaussian or tanh-sinh quadrature, is likely to lead to completely incorrect results, as the quadrature sum is (for most integrands of interest) highly ill-conditioned.
Special numerical methods which exploit the structure of the oscillation are required, an example of which is Ooura's method for Fourier integrals[6] This method attempts to evaluate the integrand at locations which asymptotically approach the zeros of the oscillation (either the sine or cosine), quickly reducing the magnitude of positive and negative terms which are summed.
^Takuya Ooura, Masatake Mori, A robust double exponential formula for Fourier-type integrals, Journal of computational and applied mathematics 112.1-2 (1999): 229-241.