(define (factor n)
  (cond
    ((negative? n)
     (cons -1 (factor (- n))))
    ((<= n 2)
     (list n))
    (else
     (let lp ((n n) (limit (inexact->exact (ceiling (sqrt n)))) (res (list)))
       (cond
         ((even? n)
          (lp (quotient n 2) (quotient limit 2) (cons 2 res)))
         ((= n 1)
          (reverse res))
         (else
          (let lp ((i 3) (n n) (limit limit) (res res))
            (cond
              ((> i limit)
               (reverse (cons n res)))
              ((zero? (remainder n i))
               (lp i (quotient n i) (quotient limit i) (cons i res)))
              (else
               (lp (+ i 2) n limit res))))))))))