Trigonometry in finite fields and a new Hartley transform

A trigonometry for finite fields is introduced. In particular, the k-trigonometric functions over the Galois field GF(q) are defined and their main properties derived. This leads to the definition of the cas/sub k/(.) function over GF(q), which in turn leads to a finite field Hartley transform (FFHT). The FFHT presented here is different from an earlier version and seems to be the more natural one.