# Euler problems/151 to 160

## Contents

## Problem 151

Paper sheets of standard sizes: an expected-value problem.

Solution:

```
problem_151 = undefined
```

## 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
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]
-- Verify that we need not consider terms divisible by 11, or by any
-- prime greater than 13. Nor need we consider any term divisible
-- by 25, 27, 32, or 49.
verify = all (\p -> null $ tail $ pfree [p, 2*p..85] 0 p) $
11 : dropWhile (< 17) primes ++ [25, 27, 32, 49]
-- 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 f [(0, [[]])] [13, 7, 5]
where
f 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 (x, _) (y, _) = x == y
cmpFst (x, _) (y, _) = compare x y
-- All subsets (of an ordered set) whose sum of inverse squares is x
findInvSq x y = f x $ zip3 y (map invSq y) (map sumInvSq $ init $ tails y)
where
f 0 _ = [[]]
f x ((n, r, s):ns)
| r > x = f x ns
| s < x = []
| otherwise = map (n :) (f (x - r) ns) ++ f x ns
f _ _ = []
-- 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 = if verify
then [sort $ u ++ v | (x, s) <- only23,
u <- findInvSq (1%2 - x) all23,
v <- s]
else undefined
problem_152 = length solutions
```

## Problem 153

Investigating Gaussian Integers

Solution:

```
problem_153 = undefined
```

## Problem 154

Exploring Pascal's pyramid.

Solution:

```
problem_154 = undefined
```

## Problem 155

Counting Capacitor Circuits.

Solution:

```
problem_155 = undefined
```

## Problem 156

Counting Digits

Solution:

```
problem_156 = undefined
```

## Problem 157

Solving the diophantine equation 1/a+1/b= p/10n

Solution:

```
problem_157 = undefined
```

## Problem 158

Exploring strings for which only one character comes lexicographically after its neighbour to the left.

Solution:

```
problem_158 = undefined
```

## Problem 159

Digital root sums of factorisations.

Solution:

```
problem_159 = undefined
```

## 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)
```