% no stack consuming version

factors(N) ->
     factors(N,2,[]).

factors(1,_,Acc) -> Acc;
factors(N,K,Acc) when N < K*K -> [N|Acc];
factors(N,K,Acc) when N rem K == 0 ->
    factors(N div K,K, [K|Acc]);
factors(N,K,Acc) ->
    factors(N,K+1,Acc).