An approximate functional form of the joint distribution p(E₁,,E_m) in P

A compact approximate formula is presented for the joint distribution p(E₁,,E_m) of m structure factors for an equal-atom structure in the space group P. The formula is based on the peculiar behaviour at infinity of suitable approximations of the characteristic function of p(E₁,,E_m). The case (E_l, E₂) = (E_2h, E_h) is considered for values |U_2h|, |U_h| ≤-0.45. The conditional probability P₊ (E_2h| E_h) that is obtained with the above method is compared with the tangent formula of Cochran & Woolfson [Acta Cryst. (1955), 8, 1-12].