Since f(13) = 6 on base 4 too, and the base was not given, if we select 4 as the base then 131 = f(131). For the math dummies: 131 in base 4 is 30 in base 10 better known as the decimal system. Proving this as the smallest solution is farly easy (try it at home). One may think that 10 in binary is a valid answer, however f(13), google's own example, is undefined under binary.