Euler problems/21 to 30

Problem 21

Evaluate the sum of all amicable pairs under 10000.

Solution:

problem_21 = undefined

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:

problem_25 = undefined

Problem 26

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

Solution:

problem_26 = undefined

Problem 27

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

Solution:

problem_27 = undefined

Problem 28

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

Solution:

problem_28 = undefined

Problem 29

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

Solution:

problem_29 = undefined

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