Difference between revisions of "Euler problems/151 to 160"
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Solution: |
Solution: |
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<haskell> |
<haskell> |
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− | problem_151 = |
+ | problem_151 = fun (1,1,1,1) |
+ | |||
+ | fun (0,0,0,1) = 0 |
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+ | fun (0,0,1,0) = fun (0,0,0,1) + 1 |
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+ | fun (0,1,0,0) = fun (0,0,1,1) + 1 |
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+ | fun (1,0,0,0) = fun (0,1,1,1) + 1 |
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+ | fun (a,b,c,d) = |
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+ | (pickA + pickB + pickC + pickD) / (a + b + c + d) |
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+ | where |
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+ | pickA | a > 0 = a * fun (a-1,b+1,c+1,d+1) |
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+ | | otherwise = 0 |
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+ | pickB | b > 0 = b * fun (a,b-1,c+1,d+1) |
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+ | | otherwise = 0 |
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+ | pickC | c > 0 = c * fun (a,b,c-1,d+1) |
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+ | | otherwise = 0 |
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+ | pickD | d > 0 = d * fun (a,b,c,d-1) |
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+ | | otherwise = 0 |
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</haskell> |
</haskell> |
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Line 18: | Line 34: | ||
import Data.Ratio |
import Data.Ratio |
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import Data.List |
import Data.List |
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+ | import Data.Ord (comparing) |
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− | |||
+ | import Data.Function (on) |
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+ | |||
invSq n = 1 % (n * n) |
invSq n = 1 % (n * n) |
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sumInvSq = sum . map invSq |
sumInvSq = sum . map invSq |
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+ | |||
− | |||
subsets (x:xs) = let s = subsets xs in s ++ map (x :) s |
subsets (x:xs) = let s = subsets xs in s ++ map (x :) s |
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subsets _ = [[]] |
subsets _ = [[]] |
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+ | |||
− | |||
primes = 2 : 3 : 7 : [p | p <- [11, 13..79], |
primes = 2 : 3 : 7 : [p | p <- [11, 13..79], |
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all (\q -> p `mod` q /= 0) [3, 5, 7]] |
all (\q -> p `mod` q /= 0) [3, 5, 7]] |
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+ | |||
− | |||
-- All subsets whose sum of inverse squares, |
-- All subsets whose sum of inverse squares, |
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-- when added to x, does not contain a factor of p |
-- when added to x, does not contain a factor of p |
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pfree s x p = [(y, t) | t <- subsets s, let y = x + sumInvSq t, |
pfree s x p = [(y, t) | t <- subsets s, let y = x + sumInvSq t, |
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denominator y `mod` p /= 0] |
denominator y `mod` p /= 0] |
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+ | |||
− | |||
+ | |||
− | -- Verify that we need not consider terms divisible by 11, or by any |
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− | -- prime greater than 13. Nor need we consider any term divisible |
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− | -- by 25, 27, 32, or 49. |
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− | verify = all (\p -> null $ tail $ pfree [p, 2*p..85] 0 p) $ |
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− | 11 : dropWhile (< 17) primes ++ [25, 27, 32, 49] |
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− | |||
-- All pairs (x, s) where x is a rational number whose reduced |
-- All pairs (x, s) where x is a rational number whose reduced |
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-- denominator is not divisible by any prime greater than 3; |
-- denominator is not divisible by any prime greater than 3; |
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-- and s is all sets of numbers up to 80 divisible |
-- and s is all sets of numbers up to 80 divisible |
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-- by a prime greater than 3, whose sum of inverse squares is x. |
-- by a prime greater than 3, whose sum of inverse squares is x. |
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− | only23 = foldl |
+ | only23 = foldl fun [(0, [[]])] [13, 7, 5] |
− | where |
+ | where |
− | + | fun a p = |
|
− | + | collect $ [(y, u ++ v) | |
|
− | + | (x, s) <- a, |
|
− | + | (y, v) <- pfree (terms p) x p, |
|
− | + | u <- s] |
|
+ | terms p = |
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− | 11 : takeWhile (>= p) [13, 7, 5] |
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− | + | [n * p | |
|
+ | n <- [1..80`div`p], |
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− | collect = map (\z -> (fst $ head z, map snd z)) . |
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− | + | all (\q -> n `mod` q /= 0) $ |
|
− | + | 11 : takeWhile (>= p) [13, 7, 5] |
|
− | + | ] |
|
+ | collect = |
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− | |||
+ | map (\z -> (fst $ head z, map snd z)) . |
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+ | groupBy fstEq . sortBy cmpFst |
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+ | fstEq = (==) `on` fst |
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+ | cmpFst = comparing fst |
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+ | |||
-- All subsets (of an ordered set) whose sum of inverse squares is x |
-- All subsets (of an ordered set) whose sum of inverse squares is x |
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− | findInvSq x y = |
+ | findInvSq x y = |
+ | fun x $ zip3 y (map invSq y) (map sumInvSq $ init $ tails y) |
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− | where |
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+ | where |
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− | f 0 _ = [[]] |
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− | + | fun 0 _ = [[]] |
|
− | + | fun x ((n, r, s):ns) |
|
− | | |
+ | | r > x = fun x ns |
− | + | | s < x = [] |
|
− | + | | otherwise = map (n :) (fun (x - r) ns) ++ fun x ns |
|
+ | fun _ _ = [] |
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− | |||
+ | |||
-- All numbers up to 80 that are divisible only by the primes |
-- All numbers up to 80 that are divisible only by the primes |
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-- 2 and 3 and are not divisible by 32 or 27. |
-- 2 and 3 and are not divisible by 32 or 27. |
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all23 = [n | a <- [0..4], b <- [0..2], let n = 2^a * 3^b, n <= 80] |
all23 = [n | a <- [0..4], b <- [0..2], let n = 2^a * 3^b, n <= 80] |
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+ | |||
− | |||
− | solutions = |
+ | solutions = |
− | + | [sort $ u ++ v | |
|
+ | (x, s) <- only23, |
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− | u <- findInvSq (1%2 - x) all23, |
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+ | u <- findInvSq (1%2 - x) all23, |
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− | v <- s] |
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+ | v <- s |
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− | else undefined |
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+ | ] |
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− | |||
+ | |||
problem_152 = length solutions |
problem_152 = length solutions |
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</haskell> |
</haskell> |
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Line 82: | Line 102: | ||
== [http://projecteuler.net/index.php?section=problems&id=153 Problem 153] == |
== [http://projecteuler.net/index.php?section=problems&id=153 Problem 153] == |
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Investigating Gaussian Integers |
Investigating Gaussian Integers |
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− | |||
− | Solution: |
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− | <haskell> |
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− | problem_153 = undefined |
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− | </haskell> |
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== [http://projecteuler.net/index.php?section=problems&id=154 Problem 154] == |
== [http://projecteuler.net/index.php?section=problems&id=154 Problem 154] == |
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Exploring Pascal's pyramid. |
Exploring Pascal's pyramid. |
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+ | {{sect-stub}} |
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− | Solution: |
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− | <haskell> |
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− | problem_154 = undefined |
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− | </haskell> |
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== [http://projecteuler.net/index.php?section=problems&id=155 Problem 155] == |
== [http://projecteuler.net/index.php?section=problems&id=155 Problem 155] == |
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Line 101: | Line 113: | ||
Solution: |
Solution: |
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<haskell> |
<haskell> |
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+ | --http://www.research.att.com/~njas/sequences/A051389 |
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− | problem_155 = undefined |
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+ | a051389= |
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+ | [1, 2, 4, 8, 20, 42, |
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+ | 102, 250, 610, 1486, |
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+ | 3710, 9228, 23050, 57718, |
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+ | 145288, 365820, 922194, 2327914 |
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+ | ] |
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+ | problem_155 = sum a051389 |
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</haskell> |
</haskell> |
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== [http://projecteuler.net/index.php?section=problems&id=156 Problem 156] == |
== [http://projecteuler.net/index.php?section=problems&id=156 Problem 156] == |
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Counting Digits |
Counting Digits |
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− | |||
− | Solution: |
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− | <haskell> |
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− | digits =reverse.digits' |
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− | where |
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− | digits' n |
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− | |n<10=[n] |
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− | |otherwise= y:digits' x |
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− | where |
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− | (x,y)=divMod n 10 |
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− | digitsToNum n=foldl dmm 0 n |
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− | where |
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− | dmm=(\x y->x*10+y) |
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− | countA :: Int -> Integer |
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− | countA 0 = 0 |
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− | countA k = fromIntegral k * (10^(k-1)) |
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− | |||
− | countFun :: Integer -> Integer -> Integer |
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− | countFun _ 0 = 0 |
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− | countFun d n = countL ds k |
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− | where |
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− | ds = digits n |
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− | k = length ds - 1 |
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− | countL [a] _ |
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− | | a < d = 0 |
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− | | otherwise = 1 |
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− | countL (a:tl) m |
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− | | a < d = a*countA m + countL tl (m-1) |
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− | | a == d = a*countA m + digitsToNum tl + 1 + countL tl (m-1) |
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− | | otherwise = a*countA m + 10^m + countL tl (m-1) |
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− | |||
− | fixedPoints :: Integer -> [Integer] |
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− | fixedPoints d |
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− | = [a*10^10+b | a <- [0 .. d-1], b <- findFrom 0 (10^10-1)] |
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− | where |
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− | fun = countFun d |
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− | good r = r == fun r |
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− | findFrom lo hi |
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− | | hi < lo = [] |
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− | | good lo = lgs ++ findFrom (last lgs + 2) hi |
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− | | good hi = findFrom lo (last hgs - 2) ++ reverse hgs |
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− | | h1 < l1 = [] |
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− | | l1 == h1 = if good l1 then [l1] else [] |
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− | | m0 == m1 = findFrom l1 (head mgs - 2) ++ mgs |
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− | ++ findFrom (last mgs + 2) h1 |
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− | | m0 < m1 = findFrom l1 (m0-1) ++ findFrom (goUp h1 m1) h1 |
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− | | otherwise = findFrom l1 (goDown l1 m1) ++ findFrom (m0+1) h1 |
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− | where |
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− | l1 = goUp hi lo |
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− | h1 = goDown l1 hi |
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− | goUp bd k |
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− | | k < k1 && k < bd = goUp bd k1 |
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− | | otherwise = k |
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− | where |
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− | k1 = fun k |
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− | goDown bd k |
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− | | k1 < k && bd < k = goDown bd k1 |
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− | | otherwise = k |
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− | where |
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− | k1 = fun k |
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− | m0 = (l1 + h1) `div` 2 |
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− | m1 = fun m0 |
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− | lgs = takeWhile good [lo .. hi] |
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− | hgs = takeWhile good [hi,hi-1 .. lo] |
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− | mgs = reverse (takeWhile good [m0,m0-1 .. l1]) |
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− | ++ takeWhile good [m0+1 .. h1] |
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− | problem_156=sum[sum $fixedPoints a|a<-[1..9]] |
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− | </haskell> |
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== [http://projecteuler.net/index.php?section=problems&id=157 Problem 157] == |
== [http://projecteuler.net/index.php?section=problems&id=157 Problem 157] == |
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Line 180: | Line 131: | ||
Solution: |
Solution: |
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<haskell> |
<haskell> |
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+ | -- Call (a,b,p) a primitive tuple of equation 1/a+1/b=p/10^n |
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− | problem_157 = undefined |
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+ | -- a and b are divisors of 10^n, gcd a b == 1, a <= b and a*b <= 10^n |
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+ | -- I noticed that the number of variants with a primitive tuple |
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+ | -- is equal to the number of divisors of p. |
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+ | -- So I produced all possible primitive tuples per 10^n and |
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+ | -- summed all the number of divisors of every p |
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+ | |||
+ | import Data.List |
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+ | k `divides` n = n `mod` k == 0 |
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+ | |||
+ | divisors n |
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+ | | n == 10 = [1,2,5,10] |
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+ | | otherwise = |
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+ | [ d | |
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+ | d <- [1..n `div` 5], |
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+ | d `divides` n ] |
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+ | ++ [n `div` 4, n `div` 2,n] |
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+ | fp n = |
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+ | [ n*(a+b) `div` ab | |
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+ | a <- ds, |
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+ | b <- dropWhile (<a) ds, |
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+ | gcd a b == 1, |
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+ | let ab = a*b, |
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+ | ab <= n |
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+ | ] |
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+ | where |
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+ | ds = divisors n |
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+ | numDivisors :: Integer -> Integer |
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+ | numDivisors n = product [ toInteger (a+1) | (p,a) <- primePowerFactors n] |
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+ | numVgln = sum . map numDivisors . fp |
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+ | |||
+ | main = do |
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+ | print . sum . map numVgln . takeWhile (<=10^9) . iterate (10*) $ 10 |
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+ | primePowerFactors x = [(head a ,length a)|a<-group$primeFactors x] |
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+ | merge xs@(x:xt) ys@(y:yt) = case compare x y of |
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+ | LT -> x : (merge xt ys) |
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+ | EQ -> x : (merge xt yt) |
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+ | GT -> y : (merge xs yt) |
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+ | |||
+ | diff xs@(x:xt) ys@(y:yt) = case compare x y of |
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+ | LT -> x : (diff xt ys) |
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+ | EQ -> diff xt yt |
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+ | GT -> diff xs yt |
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+ | |||
+ | primes, nonprimes :: [Integer] |
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+ | primes = [2,3,5] ++ (diff [7,9..] nonprimes) |
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+ | nonprimes = foldr1 f . map g $ tail primes |
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+ | where f (x:xt) ys = x : (merge xt ys) |
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+ | g p = [ n*p | n <- [p,p+2..]] |
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+ | primeFactors n = |
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+ | factor n primes |
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+ | where |
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+ | factor n (p:ps) |
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+ | | p*p > n = [n] |
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+ | | n `mod` p == 0 = p : factor (n `div` p) (p:ps) |
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+ | | otherwise = factor n ps |
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</haskell> |
</haskell> |
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Line 188: | Line 194: | ||
Solution: |
Solution: |
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<haskell> |
<haskell> |
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+ | factorial n = product [1..toInteger n] |
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− | problem_158 = undefined |
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+ | fallingFactorial x n = product [x - i | i <- [0..fromIntegral n - 1] ] |
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+ | choose n k = fallingFactorial n k `div` factorial k |
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+ | fun n=(2 ^ n - n - 1) * choose 26 n |
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+ | problem_158=maximum$map fun [1..26] |
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</haskell> |
</haskell> |
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Line 201: | Line 211: | ||
spfArray :: U.UArray Int Int |
spfArray :: U.UArray Int Int |
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spfArray = runSTUArray (do |
spfArray = runSTUArray (do |
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− | arr <- newArray ( |
+ | arr <- newArray (2,m-1) 0 |
+ | forM_ [2 .. m-1] $ \n -> |
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− | loop arr 2 |
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+ | writeArray arr n (n-9*((n-1) `div` 9)) |
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− | forM_ [2 .. m - 1] $ \ x -> |
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− | + | forM_ [2 .. m-1] $ \x -> |
|
+ | forM_ [2 .. m`div`n-1] $ \n -> |
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+ | incArray arr x n |
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return arr |
return arr |
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) |
) |
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where |
where |
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m=10^6 |
m=10^6 |
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− | loop arr n |
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− | |n>=m=return () |
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− | |otherwise=do writeArray arr n (n-9*(div (n-1) 9)) |
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− | loop arr (n+1) |
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− | loop2 arr x n |
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− | |n*x>=m=return () |
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− | |otherwise=do incArray arr x n |
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− | loop2 arr x (n+1) |
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incArray arr x n = do |
incArray arr x n = do |
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a <- readArray arr x |
a <- readArray arr x |
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Line 222: | Line 226: | ||
ab <- readArray arr (x*n) |
ab <- readArray arr (x*n) |
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when(ab<a+b) (writeArray arr (x*n) (a + b)) |
when(ab<a+b) (writeArray arr (x*n) (a + b)) |
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− | writ x=appendFile "p159.log"$ |
+ | writ x=appendFile "p159.log"$ show x ++ "\n" |
+ | main=mapM_ writ $U.elems spfArray |
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− | main=do |
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− | mapM_ writ $U.elems spfArray |
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problem_159 = main |
problem_159 = main |
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Line 307: | Line 310: | ||
hFacial 0=1 |
hFacial 0=1 |
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hFacial a |
hFacial a |
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− | |gcd a 5==1= |
+ | |gcd a 5==1=(a*hFacial(a-1)) `mod` (5^5) |
|otherwise=hFacial(a-1) |
|otherwise=hFacial(a-1) |
||
− | fastFacial a= hFacial $ |
+ | fastFacial a= hFacial $a `mod` 6250 |
− | numPrime x p=takeWhile(>0) [ |
+ | numPrime x p=takeWhile(>0) [x `div` (p^a)|a<-[1..]] |
p160 x=mulMod t5 a b |
p160 x=mulMod t5 a b |
||
where |
where |
||
t5=10^5 |
t5=10^5 |
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lst=numPrime x 5 |
lst=numPrime x 5 |
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− | a=powMod t5 1563 $ |
+ | a=powMod t5 1563 $c `mod` 2500 |
b=productMod c6 |
b=productMod c6 |
||
c=sum lst |
c=sum lst |
Latest revision as of 08:22, 23 February 2010
Problem 151
Paper sheets of standard sizes: an expected-value problem.
Solution:
problem_151 = fun (1,1,1,1)
fun (0,0,0,1) = 0
fun (0,0,1,0) = fun (0,0,0,1) + 1
fun (0,1,0,0) = fun (0,0,1,1) + 1
fun (1,0,0,0) = fun (0,1,1,1) + 1
fun (a,b,c,d) =
(pickA + pickB + pickC + pickD) / (a + b + c + d)
where
pickA | a > 0 = a * fun (a-1,b+1,c+1,d+1)
| otherwise = 0
pickB | b > 0 = b * fun (a,b-1,c+1,d+1)
| otherwise = 0
pickC | c > 0 = c * fun (a,b,c-1,d+1)
| otherwise = 0
pickD | d > 0 = d * fun (a,b,c,d-1)
| otherwise = 0
Problem 152
Writing 1/2 as a sum of inverse squares
Note that if p is an odd prime, the sum of inverse squares of all terms divisible by p must have reduced denominator not divisible by p.
Solution:
import Data.Ratio
import Data.List
import Data.Ord (comparing)
import Data.Function (on)
invSq n = 1 % (n * n)
sumInvSq = sum . map invSq
subsets (x:xs) = let s = subsets xs in s ++ map (x :) s
subsets _ = [[]]
primes = 2 : 3 : 7 : [p | p <- [11, 13..79],
all (\q -> p `mod` q /= 0) [3, 5, 7]]
-- All subsets whose sum of inverse squares,
-- when added to x, does not contain a factor of p
pfree s x p = [(y, t) | t <- subsets s, let y = x + sumInvSq t,
denominator y `mod` p /= 0]
-- All pairs (x, s) where x is a rational number whose reduced
-- denominator is not divisible by any prime greater than 3;
-- and s is all sets of numbers up to 80 divisible
-- by a prime greater than 3, whose sum of inverse squares is x.
only23 = foldl fun [(0, [[]])] [13, 7, 5]
where
fun a p =
collect $ [(y, u ++ v) |
(x, s) <- a,
(y, v) <- pfree (terms p) x p,
u <- s]
terms p =
[n * p |
n <- [1..80`div`p],
all (\q -> n `mod` q /= 0) $
11 : takeWhile (>= p) [13, 7, 5]
]
collect =
map (\z -> (fst $ head z, map snd z)) .
groupBy fstEq . sortBy cmpFst
fstEq = (==) `on` fst
cmpFst = comparing fst
-- All subsets (of an ordered set) whose sum of inverse squares is x
findInvSq x y =
fun x $ zip3 y (map invSq y) (map sumInvSq $ init $ tails y)
where
fun 0 _ = [[]]
fun x ((n, r, s):ns)
| r > x = fun x ns
| s < x = []
| otherwise = map (n :) (fun (x - r) ns) ++ fun x ns
fun _ _ = []
-- All numbers up to 80 that are divisible only by the primes
-- 2 and 3 and are not divisible by 32 or 27.
all23 = [n | a <- [0..4], b <- [0..2], let n = 2^a * 3^b, n <= 80]
solutions =
[sort $ u ++ v |
(x, s) <- only23,
u <- findInvSq (1%2 - x) all23,
v <- s
]
problem_152 = length solutions
Problem 153
Investigating Gaussian Integers
Problem 154
Exploring Pascal's pyramid.
Problem 155
Counting Capacitor Circuits.
Solution:
--http://www.research.att.com/~njas/sequences/A051389
a051389=
[1, 2, 4, 8, 20, 42,
102, 250, 610, 1486,
3710, 9228, 23050, 57718,
145288, 365820, 922194, 2327914
]
problem_155 = sum a051389
Problem 156
Counting Digits
Problem 157
Solving the diophantine equation 1/a+1/b= p/10n
Solution:
-- Call (a,b,p) a primitive tuple of equation 1/a+1/b=p/10^n
-- a and b are divisors of 10^n, gcd a b == 1, a <= b and a*b <= 10^n
-- I noticed that the number of variants with a primitive tuple
-- is equal to the number of divisors of p.
-- So I produced all possible primitive tuples per 10^n and
-- summed all the number of divisors of every p
import Data.List
k `divides` n = n `mod` k == 0
divisors n
| n == 10 = [1,2,5,10]
| otherwise =
[ d |
d <- [1..n `div` 5],
d `divides` n ]
++ [n `div` 4, n `div` 2,n]
fp n =
[ n*(a+b) `div` ab |
a <- ds,
b <- dropWhile (<a) ds,
gcd a b == 1,
let ab = a*b,
ab <= n
]
where
ds = divisors n
numDivisors :: Integer -> Integer
numDivisors n = product [ toInteger (a+1) | (p,a) <- primePowerFactors n]
numVgln = sum . map numDivisors . fp
main = do
print . sum . map numVgln . takeWhile (<=10^9) . iterate (10*) $ 10
primePowerFactors x = [(head a ,length a)|a<-group$primeFactors x]
merge xs@(x:xt) ys@(y:yt) = case compare x y of
LT -> x : (merge xt ys)
EQ -> x : (merge xt yt)
GT -> y : (merge xs yt)
diff xs@(x:xt) ys@(y:yt) = case compare x y of
LT -> x : (diff xt ys)
EQ -> diff xt yt
GT -> diff xs yt
primes, nonprimes :: [Integer]
primes = [2,3,5] ++ (diff [7,9..] nonprimes)
nonprimes = foldr1 f . map g $ tail primes
where f (x:xt) ys = x : (merge xt ys)
g p = [ n*p | n <- [p,p+2..]]
primeFactors n =
factor n primes
where
factor n (p:ps)
| p*p > n = [n]
| n `mod` p == 0 = p : factor (n `div` p) (p:ps)
| otherwise = factor n ps
Problem 158
Exploring strings for which only one character comes lexicographically after its neighbour to the left.
Solution:
factorial n = product [1..toInteger n]
fallingFactorial x n = product [x - i | i <- [0..fromIntegral n - 1] ]
choose n k = fallingFactorial n k `div` factorial k
fun n=(2 ^ n - n - 1) * choose 26 n
problem_158=maximum$map fun [1..26]
Problem 159
Digital root sums of factorisations.
Solution:
import Control.Monad
import Data.Array.ST
import qualified Data.Array.Unboxed as U
spfArray :: U.UArray Int Int
spfArray = runSTUArray (do
arr <- newArray (2,m-1) 0
forM_ [2 .. m-1] $ \n ->
writeArray arr n (n-9*((n-1) `div` 9))
forM_ [2 .. m-1] $ \x ->
forM_ [2 .. m`div`n-1] $ \n ->
incArray arr x n
return arr
)
where
m=10^6
incArray arr x n = do
a <- readArray arr x
b <- readArray arr n
ab <- readArray arr (x*n)
when(ab<a+b) (writeArray arr (x*n) (a + b))
writ x=appendFile "p159.log"$ show x ++ "\n"
main=mapM_ writ $U.elems spfArray
problem_159 = main
--at first ,make main to get file "p159.log"
--then ,add all num in the file
Problem 160
Factorial trailing digits
We use the following two facts:
Fact 1: (2^(d + 4*5^(d-1)) - 2^d) `mod` 10^d == 0
Fact 2: product [n | n <- [0..10^d], gcd n 10 == 1] `mod` 10^d == 1
We really only need these two facts for the special case of
d == 5
, and we can verify that directly by
evaluating the above two Haskell expressions.
More generally:
Fact 1 follows from the fact that the group of invertible elements
of the ring of integers modulo 5^d
has
4*5^(d-1)
elements.
Fact 2 follows from the fact that the group of invertible elements
of the ring of integers modulo 10^d
is isomorphic to the product
of a cyclic group of order 2 and another cyclic group.
Solution:
problem_160 = trailingFactorialDigits 5 (10^12)
trailingFactorialDigits d n = twos `times` odds
where
base = 10 ^ d
x `times` y = (x * y) `mod` base
multiply = foldl' times 1
x `toPower` k = multiply $ genericReplicate n x
e = facFactors 2 n - facFactors 5 n
twos
| e <= d = 2 `toPower` e
| otherwise = 2 `toPower` (d + (e - d) `mod` (4 * 5 ^ (d - 1)))
odds = multiply [odd | a <- takeWhile (<= n) $ iterate (* 2) 1,
b <- takeWhile (<= n) $ iterate (* 5) a,
odd <- [3, 5 .. n `div` b `mod` base],
odd `mod` 5 /= 0]
-- The number of factors of the prime p in n!
facFactors p = sum . zipWith (*) (iterate (\x -> p * x + 1) 1) .
tail . radix p
-- The digits of n in base b representation
radix p = map snd . takeWhile (/= (0, 0)) .
iterate ((`divMod` p) . fst) . (`divMod` p)
it have another fast way to do this .
Solution:
import Data.List
mulMod :: Integral a => a -> a -> a -> a
mulMod a b c= (b * c) `rem` a
squareMod :: Integral a => a -> a -> a
squareMod a b = (b * b) `rem` a
pow' :: (Num a, Integral b) => (a -> a -> a) -> (a -> a) -> a -> b -> a
pow' _ _ _ 0 = 1
pow' mul sq x' n' = f x' n' 1
where
f x n y
| n == 1 = x `mul` y
| r == 0 = f x2 q y
| otherwise = f x2 q (x `mul` y)
where
(q,r) = quotRem n 2
x2 = sq x
powMod :: Integral a => a -> a -> a -> a
powMod m = pow' (mulMod m) (squareMod m)
productMod =foldl (mulMod (10^5)) 1
hFacial 0=1
hFacial a
|gcd a 5==1=(a*hFacial(a-1)) `mod` (5^5)
|otherwise=hFacial(a-1)
fastFacial a= hFacial $a `mod` 6250
numPrime x p=takeWhile(>0) [x `div` (p^a)|a<-[1..]]
p160 x=mulMod t5 a b
where
t5=10^5
lst=numPrime x 5
a=powMod t5 1563 $c `mod` 2500
b=productMod c6
c=sum lst
c6=map fastFacial $x:lst
problem_160 = p160 (10^12)