(define mod-expt
(lambda (base exponent modulus)
(define mod*
(lambda (x y)
(remainder (* x y) modulus)))
(if (= exponent 0)
1
(if (even? exponent)
(let ((x (mod-expt base (/ exponent 2) modulus)))
(mod* x x))
(mod* (mod-expt base (- exponent 1) modulus)
base)))))
You need to make this logarithmic iterative, using the fact that when
e is even: