Another Project Euler problem:

2^15 = 32768 and the sum of its digits is 3 + 2 + 7 + 6 + 8 = 26.
What is the sum of the digits of the number 2^1000?

Here ^ denotes exponentiation. And there’s the first rub. The Erlang math:pow/2 function returns a floating point approximation, so we need to code up an exact integer operation. 

-module(mymath).
-export([pow/2]).
 square(X) ->
    X * X.
 pow(_, 0) -> 1;
 pow(N, 1) -> N;
 pow(N,K) when K rem 2 == 0 -> square(pow(N, K div 2));
 pow(N,K) -> N * pow(N, K-1).

Then we can call it from the command line: 

21> D = integer_to_list(mymath:pow(2,1000)).
"10715086071862673209484250490600018105614048117055336074437503883703510511249361224931983788156958581275946729175531468251871452856923140435984577574698574803934567774824230985421074605062371141877954182153046474983581941267398767559165543946077062914571196477686542167660429831652624386837205668069376"

My initial solution (wrong!) was 

22> lists:sum(D).
15862

Recall that Erlang characters are really integers, where $0 == 48.
We just need an offset.

23> lists:sum(D) - length(D)*$0.
1366