# nayuki/Project-Euler-solutions

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 {- - Solution to Project Euler problem 120 - Copyright (c) Project Nayuki. All rights reserved. - - https://www.nayuki.io/page/project-euler-solutions - https://github.com/nayuki/Project-Euler-solutions -} {- - For a given a, what is the n that maximizes the remainder, and what is the value of this remainder? - - Let's simplify one term, mod a^2: - (a+1)^n = 1^n + (n choose 1) a 1^(n-1) + (n choose 2) a^2 1^(n-2) + ... (by the binomial theorem) - = 1 + an + 0. (remaining addends are 0 because they have a to the power of 2 or more, mod a^2) - Similarly for the other term, mod a^2: - (a-1)^n = (-1)^n + (n choose 1) a (-1)^(n-1) + ... - = (-1)^(n-1) (-1 + an + 0) - = if n is even then (1 - an) else (an - 1). - Therefore, adding the two terms: - (a+1)^n + (a-1)^n - = if n is even then 2 else 2an. - - We can always make 2an >= 2 by taking n=1, for example. So we can disregard the "n is even" case. - Maximizing 2an mod a^2 for n is the same as maximizing 2n mod a for n. - If a is even, then the maximum achievable value is a - 2 by setting n = a/2 - 1. - Else a is odd, then the maximum achievable value is a - 1 by setting n = (a - 1) / 2. - - In conclusion, if a is even, the maximum remainder is a(a-2); - otherwise a is odd, the maximum remainder is a(a-1). -} main = putStrLn (show ans) ans = sum [a * (a - (if (mod a 2) == 0 then 2 else 1)) | a <- [3..1000]]