;;; Euler's totient function phi(m) is defined as the number
;;; of positive integers r (1 <= r <= m) that are coprime to m.

(defun phi (m)
  (if (< 0 m)
      (phi-aux 1 m)
    'undef
    )
  )

;;;  Function phi-aux(k,m) is defined as the number
;;; of positive integers r (k <= r < m) that are coprime to m.

(defun phi-aux (k m)
  (if (<= k m)
      (+ (phi-aux (1+ k) m)
	 (if (coprime k m) 1 0)
	 )
    0
    )
  )