pSum a1 d lim =
            let n1 = (lim `div` d)
                n2 = ((lim - a1) `div` d + 1)
            in if (a1 == d)
                  then (d * n1 * (n1 + 1)) `div` 2
                  else  (n2 `div` 2) * (a1 + aN(a1, d, n2))