# Euler problems/61 to 70

## 1 Problem 61

Find the sum of the only set of six 4-digit figurate numbers with a cyclic property.

Solution:

```import Data.List

triangle   = [n*(n+1)`div`2   | n <- [1..]]
square     = [n^2             | n <- [1..]]
pentagonal = [n*(3*n-1)`div`2 | n <- [1..]]
hexagonal  = [n*(2*n-1)       | n <- [1..]]
heptagonal = [n*(5*n-3)`div`2 | n <- [1..]]
octagonal  = [n*(3*n-2)       | n <- [1..]]

triangle4   = fourDigs triangle
square4     = fourDigs square
pentagonal4 = fourDigs pentagonal
hexagonal4  = fourDigs hexagonal
heptagonal4 = fourDigs heptagonal
octagonal4  = fourDigs octagonal

fourDigs = takeWhile (<10000) . dropWhile (<1000)

solve = do
(l1:l2:l3:l4:l5:l6:_) <- permute [triangle4, square4, pentagonal4, hexagonal4, heptagonal4, octagonal4]
a <- l1
let m = filter (g a) l2
b <- m
let n = filter (g b) l3
c <- n
let o = filter (g c) l4
d <- o
let p = filter (g d) l5
e <- p
let q = filter (g e) l6
f <- q
if g f a then return (sum [a,b,c,d,e,f]) else fail "burp"
where
g x y = x `mod` 100 == y `div` 100

permute        :: [a] -> [[a]]
permute []      = [[]]
permute list = concat \$ map (\(x:xs) -> map (x:) (permute xs)) (take (length list) (unfoldr (\x -> Just (x, tail x ++ [head x])) list))

## 2 Problem 62

Find the smallest cube for which exactly five permutations of its digits are cube.

Solution:

`problem_62 = undefined`

## 3 Problem 63

How many n-digit positive integers exist which are also an nth power?

Solution: Since dn has at least n+1 digits for any d≥10, we need only consider 1 through 9. If dn has fewer than n digits, every higher power of d will also be too small since d < 10. We will also never have n+1 digits for our nth powers. All we have to do is check dn for each d in {1,...,9}, trying n=1,2,... and stopping when dn has fewer than n digits.

```problem_63 = length . concatMap (takeWhile (\(n,p) -> n == nDigits p))
\$ [powers d | d <- [1..9]]
where powers d = [(n, d^n) | n <- [1..]]
nDigits n = length (show n)```

## 4 Problem 64

How many continued fractions for N ≤ 10000 have an odd period?

Solution:

`problem_64 = undefined`

## 5 Problem 65

Find the sum of digits in the numerator of the 100th convergent of the continued fraction for e.

Solution:

```import Data.Ratio

problem_65 = dsum . numerator . contFrac . take 100 \$ e
where dsum 0 = 0
dsum n = let ( d, m ) = n `divMod` 10 in m + ( dsum d )
contFrac = foldr1 (\x y -> x + 1/y)
e = 2 : 1 : insOnes [2,4..]
insOnes (x:xs) = x : 1 : 1 : insOnes xs```

## 6 Problem 66

Investigate the Diophantine equation x2 − Dy2 = 1.

Solution:

`problem_66 = undefined`

## 7 Problem 67

Using an efficient algorithm find the maximal sum in the triangle?

Solution:

```import System.Process
import IO

slurpURL url = do
(_,out,_,_) <- runInteractiveCommand \$ "curl " ++ url
hGetContents out

problem_67 = do
src <- slurpURL "http://projecteuler.net/project/triangle.txt"
print \$ head \$ foldr1 g \$ parse src
where
parse :: String -> [[Int]]
parse s = map ((map read).words) \$ lines s
f x y z = x + max y z
g xs ys = zipWith3 f xs ys \$ tail ys```

## 8 Problem 68

What is the maximum 16-digit string for a "magic" 5-gon ring?

Solution:

`problem_68 = undefined`

## 9 Problem 69

Find the value of n ≤ 1,000,000 for which n/φ(n) is a maximum.

Solution:

`problem_69 = undefined`

## 10 Problem 70

Investigate values of n for which φ(n) is a permutation of n.

Solution:

`problem_70 = undefined`