45.clj

(def triangles
     (map #(/ (* %
                 (inc %))
              2)
          (iterate inc 1)))

(def pentagonals 
     (map #(/ (* %
                 (- (* 3 %) 1))
              2)
          (iterate inc 1)))

(def hexagonals
     (map #(* %
              (- (* 2 %) 1))
	  (iterate inc 1)))

(defn incsmallest [somevec]
  (def smallest (apply min (map first somevec)))
  (map #(if (= (first %)
               smallest)
          (next %)
          (identity %))
       somevec))

(def results
     (map ffirst
          (filter #(= (first (nth % 0))
                      (first (nth % 1))
                      (first (nth % 2)))
                  (iterate incsmallest
                           [triangles pentagonals hexagonals]))))

(time (println (take 1 (drop-while #(< % 40756) results))))