Exact conditional joint probability distribution of a three-phase invariant in space group P2. I. Derivation of the Fourier coefficients

We extend our study of the conditional probability density function (c.p.d.f.) of the three-phase invariant for the space group P1 [Shmueli, Rabinovich & Weiss (1989). Acta Cryst. A45, 361-367] to the monoclinic space group P2. A detailed derivation of the characteristic function (and hence Fourier coefficients) of the latter c.p.d.f. is presented in this paper, as well as some simplifications of the resulting expressions.