Merging in 6 year old repo of project euler files

[sandbox] / euler / clj / 21.clj

(defn factors [n]
        (filter #(zero? (mod n %))
                (take (- n 1) (iterate inc 1))))

(defn d [n]
        (reduce + (factors n)))

(defn amicable? [a b]
        (and
                (= a (d b))
                (= (d a) b)))

(defn list-length [a]
        (if (= (first a) nil)
                0
                (+ 1 (list-length (rest a)))))