Problem 221

221 Alexandrian Integers

Problem 221 – Project Euler

似た問題が前にもあった.それは,n^2の約数が鍵になっていたが…

今回はn^2+1の約数が鍵になる.約数を求めるのは結構大変.

とりあえず,素因数分解を利用してみた.

{-# OPTIONS_GHC -fbang-patterns #-}
import Control.Monad (when, guard)
import Control.Monad.ST (ST, runST)
import Data.Array.ST (STUArray, newArray, readArray, writeArray)
import Data.List (sort, group)
alexandrian :: Integer -> [Integer]
alexandrian n = do
let u = floor $ fromInteger (div n 4) ** (1/3)
primes = primesUpTo u
p <- [1..u]
let v = min p.floor.sqrt.fromInteger.div n $ 4 * p
d <- takeWhile (<= v).divisors primes $ p^2 + 1
let a = p * (p+d) * (p + div (p^2+1) d)
guard $ a <= n
return a
main = print.(!!149999).sort.alexandrian $ 2 * 10^15
divisors :: [Integer] -> Integer -> [Integer]
divisors ps = sort.map product.sequence.map (scanl (*) 1).group.factors ps
factors :: [Integer] -> Integer -> [Integer]
factors ps n = f n ps
where f _ [] = []
f m (p:ps) | p*p > m = [m]
| m `mod` p ==  = p:f (div m p) (p:ps)
| otherwise = f m ps
primesUpTo :: Integer -> [Integer]
primesUpTo n =
runST $ do
p <- newArray (1,div n 2) True :: ST s (STUArray s Integer Bool)
let u = floor.sqrt.fromIntegral $ div n 2
loop !i = when (i <= u) $ sieve i (2*i*(i+1)) >> loop (i+1)
sieve !i !j = when (j <= div n 2) $ writeArray p j False >> sieve i (j+2*i+1)
primes !i !ps | i > div n 2 = return.reverse $ ps
| otherwise   = do q <- readArray p i
if q then primes (i+1) (2*i+1:ps)
else primes (i+1) ps
loop 1
primes 1 [2]

引数が 2 * 10^15 なのは度重なる実験の結果.ずるいといえば,ずるい.

More Reading
Newer// Problem 220
Older// Problem 222