Erlang: Fibonacci with Tail recursion + determining if an integer is perfect.

recursion.erl

-module(recursion).
-export([fib/1, fibtail/1, fibtail/3, isPerfect/1]).

fib(0) ->
    0;
fib(1) ->
    1;
fib(N) when N > 0 ->
    fib(N-1)+fib(N-2).

% Fibonacci with tail recursion
% C = decounter (1-N). C == 0 is the stop condition
% Acc1 = accumulator for N-1
% Acc2 = accumulator for N-2
fibtail(N) ->
    fibtail(N, 0, 1).

fibtail(0, Acc1, _) ->
    Acc1;
fibtail(C, Acc1, Acc2) ->
    fibtail(C-1, Acc1+Acc2, Acc1).

% A number is perfect when it is the sum of its divisors
isPerfect(N) ->
    isPerfect(0, 1, N).

isPerfect(Sum, _, N) when Sum == N ->
    true;
isPerfect(Sum, _, N) when Sum > N ->
    false;
isPerfect(Sum, Div, N) ->
    isPerfect(Sum + Div, nextDivisor(Div, N), N).

nextDivisor(Div, N) when N rem (Div+1) == 0 ->
    Div + 1;
nextDivisor(Div, N) when Div < N ->
    nextDivisor(Div + 1, N);
nextDivisor(Div, N) ->
    Div.