y = dct (x, n)
Computes the inverse discrete cosine transform of x.  If n is
given, then x is padded or trimmed to length n before computing
the transform. If x is a matrix, compute the transform along the
columns of the the matrix. The transform is faster if x is
real-valued and even length.

The inverse discrete cosine transform x of X can be defined as follows:

N-1
x[n] = sum w(k) X[k] cos (pi (2n-1) k / 2N ),  k = 0, ..., N-1
k=0

with w(0) = sqrt(1/N) and w(k) = sqrt(2/N), k = 1, ..., N-1

See also: idct, dct2, idct2, dctmtx