(define (ramanujan  n)
  (let lp ((w 0) (b0 1) (n n))
    (if (> n 0)
        (let ((w (+ w 1)))
          (let lp2 ((b0 b0))
            (if (< (+ 1 (* b0 b0 b0)) w)
                (lp2 (+ b0 1))
                (let lp3 ((a 1) (a3 1) (b b0) (b3 (* b0 b0 b0)) (count 0))
                  (if (<= a b)
                      (let ((s (+ a3 b3)))
                        (cond
                         ((< s w)
                          (let ((aa (+ a 1)))
                            (lp3 aa (* aa aa aa) b b3 count)))
                         ((= s w)
                          (let ((count (+ count 1)))
                            (if (> count 1)
                                (lp w b0 (- n 1))
                                (let* ((b   (- b 1))
                                       (b3  (* b b b)))
                                  (lp3 a a3 b b3 count)))))
                         (else
                          (let* ((b   (- b 1))
                                 (b3  (* b b b)))
                            (lp3 a a3 b b3 count)))))
                      (lp w b0 n))))))
        w)))

(pk (ramanujan 20))