Euler problems/21 to 30

Problem 21

Evaluate the sum of all amicable pairs under 10000.

Solution: This is a little slow because of the naive method used to compute the divisors.

```problem_21 = sum [m+n | m <- [2..9999], let n = divisorsSum ! m, amicable m n]
where amicable m n = m < n && n < 10000 && divisorsSum ! n == m
[(i, sum (divisors i)) | i <- [1..9999]]
divisors n = [j | j <- [1..n `div` 2], n `mod` j == 0]
```

Problem 22

What is the total of all the name scores in the file of first names?

Solution:

```-- apply to a list of names
problem_22 :: [String] -> Int
problem_22 = sum . zipWith (*) [ 1 .. ] . map score
where score = sum . map ( subtract 64 . ord )
```

Problem 23

Find the sum of all the positive integers which cannot be written as the sum of two abundant numbers.

Solution:

```problem_23 = undefined
```

Problem 24

What is the millionth lexicographic permutation of the digits 0, 1, 2, 3, 4, 5, 6, 7, 8 and 9?

Solution:

```perms [] = [[]]
perms xs = do
x <- xs
map ( x: ) ( perms . delete x \$ xs )

problem_24 = ( perms "0123456789" ) !! 999999
```

Problem 25

What is the first term in the Fibonacci sequence to contain 1000 digits?

Solution:

```valid ( i, n ) = length ( show n ) == 1000

problem_25 = fst . head . filter valid . zip [ 1 .. ] \$ fibs
where fibs = 1 : 1 : 2 : zipWith (+) fibs ( tail fibs )
```

Problem 26

Find the value of d < 1000 for which 1/d contains the longest recurring cycle.

Solution:

```problem_26 = fst \$ maximumBy (\a b -> snd a `compare` snd b)
[(n,recurringCycle n) | n <- [1..999]]
where  recurringCycle d = remainders d 10 []
remainders d 0 rs = 0
remainders d r rs = let r' = r `mod` d
in case findIndex (== r') rs of
Just i  -> i + 1
Nothing -> remainders d (10*r') (r':rs)
```

Problem 27

Find a quadratic formula that produces the maximum number of primes for consecutive values of n.

Solution:

The following is written in literate Haskell:

```> import Data.List

To be sure we get the maximum type checking of the compiler,
we switch off the default type

> default ()

Generate a list of primes.
It works by filtering out numbers that are
divisable by a previously found prime

> primes :: [Int]
> primes = sieve (2 : [3, 5..])
>   where
>     sieve (p:xs) = p : sieve (filter (\x -> x `mod` p > 0) xs)

> isPrime :: Int -> Bool
> isPrime x = x `elem` (takeWhile (<= x) primes)

The lists of values we are going to try for a and b;
b must be a prime, as n² + an + b is equal to b when n = 0

> testRangeA :: [Int]
> testRangeA = [-1000 .. 1000]

> testRangeB :: [Int]
> testRangeB = takeWhile (< 1000) primes

The search

> bestCoefficients :: (Int, Int, Int)
> bestCoefficients =
>   maximumBy (\(x, _, _) (y, _, _) -> compare x y)  \$
>   [f a b | a <- testRangeA, b <- testRangeB]
>     where

Generate a list of results of the quadratic formula
(only the contiguous primes)
wrap the result in a triple, together with a and b

>       f :: Int -> Int -> (Int, Int, Int)
>       f a b = ( length \$ contiguousPrimes a b
>               , a
>               , b
>               )

> contiguousPrimes :: Int -> Int -> [Int]
> contiguousPrimes a b = takeWhile isPrime (map (quadratic a b) [0..])

> quadratic :: Int -> Int -> Int -> Int
> quadratic a b n  = n * n + a * n + b

> problem_27 =
>   do
>     let (l, a, b) = bestCoefficients
>
>     putStrLn \$ ""
>     putStrLn \$ "Problem Euler 27"
>     putStrLn \$ ""
>     putStrLn \$ "The best quadratic formula found is:"
>     putStrLn \$ "  n * n + " ++ show a ++ " * n + " ++ show b
>     putStrLn \$ ""
>     putStrLn \$ "The number of primes is: " ++ (show l)
>     putStrLn \$ ""
>     putStrLn \$ "The primes are:"
>     print \$ take l \$ contiguousPrimes a b
>     putStrLn \$ ""
```

Problem 28

What is the sum of both diagonals in a 1001 by 1001 spiral?

Solution:

```corners :: Int -> (Int, Int, Int, Int)
corners i = (n*n, 1+(n*(2*m)), 2+(n*(2*m-1)), 3+(n*(2*m-2)))
where m = (i-1) `div` 2
n = 2*m+1

sumcorners :: Int -> Int
sumcorners i = a+b+c+d where (a, b, c, d) = corners i

sumdiags :: Int -> Int
sumdiags i | even i    = error "not a spiral"
| i == 3    = s + 1
| otherwise = s + sumdiags (i-2)
where s = sumcorners i

problem_28 = sumdiags 1001
```

Problem 29

How many distinct terms are in the sequence generated by ab for 2 ≤ a ≤ 100 and 2 ≤ b ≤ 100?

Solution:

```problem_29 = length . group . sort \$ [a^b | a <- [2..100], b <- [2..100]]
```

Problem 30

Find the sum of all the numbers that can be written as the sum of fifth powers of their digits.

Solution:

```problem_30 = undefined
```