(defn tri*
  "Generates lazy sequence of triangular number"
  ([] (tri* 0 1))
  ([sum n]
   (let [new-sum (+ sum n)]
     (cons new-sum (lazy-seq (tri* new-sum (inc n)))))))

(def tri (tri*))

(defn triangular?
  "Is the number traingular? e.g. 1, 3, 6"
  [n]
  (= n (last (take-while #(>= n %) tri))))

(dotimes [n 1000]
  (println (triangular? (rand-int 500000000))))