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+;;;; ;;;;;;;;
+;;;; Largest prime factor
+;;;; Problem 3
+;;;;
+;;;; The prime factors of 13195 are 5, 7, 13 and 29.
+;;;; What is the largest prime factor of the number 600851475143 ?
+;;;; ;;;;;;;;
+
+(defconstant +big-num+ 600851475143)
+
+(defun is-prime-rec (n d)
+(cond
+  ((= 0 (mod n d)) nil)
+  ((= 0 (mod n (+ d 2))) nil)
+  ((> (* d d) n) t)
+  (t (is-prime-rec n (+ d 6)))))
+
+(defun is-prime (n)
+  (when (or (= 0 (mod n 2)) (= 0 (mod n 3))) (return-from is-prime nil))
+  (is-prime-rec n 5))
+
+
+; (print (is-prime 1597))
+
+(defvar *largest* 0)
+
+(dotimes (x +big-num+)
+  (print x)
+  (when (and (= 0 (mod +big-num+ is-prime x) (setf *largest* x)))
+
+(print *largest*)