o a fourth storage means; (g) calculating a value of f(un+1.sup.(1), . . . , un+1.sup.(k)) when a coordinate value un+1.sup.(i) (1???i???k) is stored into the fourth storage by the step (f); (h) iterating the step (b) to (g) so as to accumulate a value of generated f(un+1.sup.(1), . . . , un+1.sup.(k)) from n=1 to n=N by an accumulator, wherein the N is a predetermined integer greater than 1; and