;; helpers
(define (sieve n)
(define (aux u v)
(let ((p (car v)))
(if (> (* p p) n)
(let rev-append ((u u) (v v))
(if (null? u) v (rev-append (cdr u) (cons (car u) v))))
(aux (cons p u)
(let wheel ((u '()) (v (cdr v)) (a (* p p)))
(cond ((null? v) (reverse u))
((= (car v) a) (wheel u (cdr v) (+ a p)))
((> (car v) a) (wheel u v (+ a p)))
(else (wheel (cons (car v) u) (cdr v) a))))))))
(aux '(2)
(let range ((v '()) (k (if (odd? n) n (- n 1))))
(if (< k 3) v (range (cons k v) (- k 2))))))
(define (nth n l)
(if (or (> n (length l)) (< n 0))
(error "Index out of bounds.")
(if (eq? n 0)
(car l)
(nth (- n 1) (cdr l)))))
;; e7
(nth 10000 (sieve 110000))