recursion.erl

-module(recursion).
-export([fib/1, perfect/1]).

fib(N) when N > 0 ->
  fib(N, 0, 1).
fib(1, First, _Second) ->
  First;
fib(2, _First, Second) ->
  Second;
fib(N, First, Second) ->
  fib(N-1, Second, First+Second).

perfect(N) ->
  perfect(N, 1, 0).
perfect(N, I, Sum) when I > (N div 2) ->
  N == Sum;
perfect(N, I, Sum) when (N rem I) == 0 ->
  perfect(N, I+1, Sum + I);
perfect(N, I, Sum) ->
  perfect(N, I+1, Sum).