http://projecteuler.net/index.php?section=problems&id=14 Haskellで書くとこんな感じ。 import Data.List next n = if mod n 2 == 0 then div n 2 else 3 * n + 1 chain_length n = head [ k | (k,m) <- zip [1..] (iterate next n), m == 1 ] limit = 10…