(define (totient n)
  (let ((limit (exact (ceiling (sqrt n)))))
    (let lp ((i 2) (count 1))
      (cond ((> i limit)
             (if (= count (- i 1))
                 (- n 1)                ; shortcut for prime
                 (let lp ((i i) (count count))
                   (cond ((>= i n) count)
                         ((= 1 (gcd n i)) (lp (+ i 1) (+ count 1)))
                         (else (lp (+ i 1) count))))))
            ((= 1 (gcd n i)) (lp (+ i 1) (+ count 1)))
            (else (lp (+ i 1) count))))))