euler12.clj

; Problem 12
; 
; What is the value of the first triangle number to have over five hundred divisors?
; 

(def natnums (iterate inc 1))
(def trinums (lazy-cat [(first natnums)] (map + trinums (rest natnums))))

(defn divcount [n]
  (* 2 (count
	(for [x (range 1 (Math/sqrt n))
	      :when (zero? (rem n x))] x))))

(defn answer [n] 
     (first (filter #(> (divcount %) n) trinums)))

; => 76576500 (575 divisors)