Difference between revisions of "Euler problems/61 to 70"
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− | + | == [http://projecteuler.net/index.php?section=problems&id=61 Problem 61] == | |
+ | Find the sum of the only set of six 4-digit figurate numbers with a cyclic property. | ||
+ | |||
+ | Solution: | ||
+ | <haskell> | ||
+ | import Data.List | ||
+ | |||
+ | permute [] = [[]] | ||
+ | permute xs = concatMap (\x -> map (x:) $ permute $ delete x xs) xs | ||
+ | |||
+ | figurates n xs = extract $ concatMap (gather (map poly xs)) $ map (:[]) $ poly n | ||
+ | where gather [xs] (v:vs) | ||
+ | = let v' = match xs v | ||
+ | in if v' == [] then [] else map (:v:vs) v' | ||
+ | gather (xs:xss) (v:vs) | ||
+ | = let v' = match xs v | ||
+ | in if v' == [] then [] else concatMap (gather xss) $ map (:v:vs) v' | ||
+ | match xs (_,v) = let p = (v `mod` 100)*100 in sublist (p+10,p+100) xs | ||
+ | sublist (s,e) = takeWhile (\(_,x) -> x<e) . dropWhile (\(_,x) -> x<s) | ||
+ | link ((_,x):xs) = x `mod` 100 == (snd $ last xs) `div` 100 | ||
+ | diff (x:y:xs) = if fst x /= fst y then diff (y:xs) else False | ||
+ | diff [x] = True | ||
+ | extract = filter diff . filter link | ||
+ | poly m = [(n, x) | (n, x) <- zip [1..] $ takeWhile (<10000) | ||
+ | $ scanl (+) 1 [m-1,2*m-3..], | ||
+ | 1010 < x, x `mod` 100 > 9] | ||
+ | |||
+ | problem_61 = sum $ map snd $ head $ concatMap (figurates 3) $ permute [4..8] | ||
+ | </haskell> | ||
+ | |||
+ | == [http://projecteuler.net/index.php?section=problems&id=62 Problem 62] == | ||
+ | Find the smallest cube for which exactly five permutations of its digits are cube. | ||
+ | |||
+ | Solution: | ||
+ | <haskell> | ||
+ | import Data.List | ||
+ | import Data.Maybe | ||
+ | a = map (^3) [0..10000] | ||
+ | b = map (sort . show) a | ||
+ | c = (filter ((==5) . length) . group . sort) b | ||
+ | d = findIndex (==(head (head c))) b | ||
+ | problem_62 = (toInteger (fromJust d))^3 | ||
+ | </haskell> | ||
+ | |||
+ | == [http://projecteuler.net/index.php?section=problems&id=63 Problem 63] == | ||
+ | How many n-digit positive integers exist which are also an nth power? | ||
+ | |||
+ | Solution: | ||
+ | <haskell> | ||
+ | problem_63=length[x^y|x<-[1..9],y<-[1..22],y==(length$show$x^y)] | ||
+ | </haskell> | ||
+ | |||
+ | == [http://projecteuler.net/index.php?section=problems&id=64 Problem 64] == | ||
+ | How many continued fractions for N ≤ 10000 have an odd period? | ||
+ | |||
+ | Solution: | ||
+ | <haskell> | ||
+ | import Data.List | ||
+ | |||
+ | problem_64 =length $ filter id $ map solve $ [2..9999] \\ (map (^2) [2..100]) | ||
+ | |||
+ | solve n = even $ length $ cont n 0 1 | ||
+ | |||
+ | cont :: Int -> Int -> Int -> [Int] | ||
+ | cont r n d = m : rest | ||
+ | where | ||
+ | m = truncate ((sqrt (fromIntegral r) + fromIntegral n ) / fromIntegral d) | ||
+ | a = n - d * m | ||
+ | rest = if d == 1 && n /= 0 | ||
+ | then [] | ||
+ | else cont r (-a) ((r - a ^ 2) `div` d) | ||
+ | </haskell> | ||
+ | |||
+ | == [http://projecteuler.net/index.php?section=problems&id=65 Problem 65] == | ||
+ | Find the sum of digits in the numerator of the 100th convergent of the continued fraction for e. | ||
+ | |||
+ | Solution: | ||
+ | <haskell> | ||
+ | import Data.Char | ||
+ | import Data.Ratio | ||
+ | |||
+ | e = [2] ++ concat [ [1, 2*i, 1] | i <- [1..] ] | ||
+ | |||
+ | fraction [x] = x%1 | ||
+ | fraction (x:xs) = x%1 + 1/(fraction xs) | ||
+ | |||
+ | problem_65 = sum $ map digitToInt $ show $ numerator $ fraction $ take 100 e | ||
+ | </haskell> | ||
+ | |||
+ | == [http://projecteuler.net/index.php?section=problems&id=66 Problem 66] == | ||
+ | Investigate the Diophantine equation x<sup>2</sup> − Dy<sup>2</sup> = 1. | ||
+ | |||
+ | Solution: | ||
+ | <haskell> | ||
+ | intSqrt :: Integral a => a -> a | ||
+ | intSqrt n | ||
+ | | n < 0 = error "intSqrt: negative n" | ||
+ | | otherwise = f n | ||
+ | where | ||
+ | f x = if y < x then f y else x | ||
+ | where y = (x + (n `quot` x)) `quot` 2 | ||
+ | problem_66 = | ||
+ | snd$maximum [ (x,d) | | ||
+ | d <- [1..1000], | ||
+ | let b = intSqrt d, | ||
+ | b*b /= d, -- d can't be a perfect square | ||
+ | let (x,_) = pell d b b | ||
+ | ] | ||
+ | |||
+ | pell d wd b = piter d wd b 0 1 0 1 1 0 | ||
+ | piter d wd b i c l k m n | ||
+ | | cn == 1 = (x, y) | ||
+ | | otherwise = piter d wd bn (i+1) cn k u n v | ||
+ | where | ||
+ | yb = (wd+b) `div` c | ||
+ | bn = yb*c-b | ||
+ | cn = (d-(bn*bn)) `div` c | ||
+ | yn | i == 0 = wd | ||
+ | | otherwise = yb | ||
+ | u = k*yn+l -- u/v is the i-th convergent of sqrt(d) | ||
+ | v = n*yn+m | ||
+ | (x,y) | odd (i+1) = (u*u+d*v*v, 2*u*v) | ||
+ | | otherwise = (u,v) | ||
+ | </haskell> | ||
+ | |||
+ | == [http://projecteuler.net/index.php?section=problems&id=67 Problem 67] == | ||
+ | Using an efficient algorithm find the maximal sum in the triangle? | ||
+ | |||
+ | Solution: | ||
+ | <haskell> | ||
+ | problem_67 = readFile "triangle.txt" >>= print . solve . parse | ||
+ | parse = map (map read . words) . lines | ||
+ | solve = head . foldr1 step | ||
+ | step [] [z] = [z] | ||
+ | step (x:xs) (y:z:zs) = x + max y z : step xs (z:zs) | ||
+ | </haskell> | ||
+ | |||
+ | == [http://projecteuler.net/index.php?section=problems&id=68 Problem 68] == | ||
+ | What is the maximum 16-digit string for a "magic" 5-gon ring? | ||
+ | |||
+ | Solution: | ||
+ | <haskell> | ||
+ | import Data.List | ||
+ | permute [] = [[]] | ||
+ | permute list = | ||
+ | concat $ map (\(x:xs) -> map (x:) (permute xs)) | ||
+ | (take (length list) | ||
+ | (unfoldr (\x -> Just (x, tail x ++ [head x])) list)) | ||
+ | problem_68 = | ||
+ | maximum $ map (concat . map show) poel | ||
+ | where | ||
+ | gon68 = [1..10] | ||
+ | knip = (length gon68) `div` 2 | ||
+ | (is,es) = splitAt knip gon68 | ||
+ | extnodes = map (\x -> [head es]++x) $ permute $ tail es | ||
+ | intnodes = map (\(p:ps) -> zipWith (\ x y -> [x]++[y]) | ||
+ | (p:ps) (ps++[p])) $ permute is | ||
+ | poel = [ concat hs | hs <- [ zipWith (\x y -> [x]++y) uitsteeksels organen | | ||
+ | uitsteeksels <- extnodes, organen <- intnodes ], | ||
+ | let subsom = map (sum) hs, length (nub subsom) == 1 ] | ||
+ | </haskell> | ||
+ | |||
+ | == [http://projecteuler.net/index.php?section=problems&id=69 Problem 69] == | ||
+ | Find the value of n ≤ 1,000,000 for which n/φ(n) is a maximum. | ||
+ | |||
+ | Solution: | ||
+ | <haskell> | ||
+ | {-phi(n) = n*(1-1/p1)*(1-1/p2)*...*(1-1/pn) | ||
+ | n/phi(n) = 1/(1-1/p1)*(1-1/p2)*...*(1-1/pn) | ||
+ | (1-1/p) will be minimal for a small p and 1/(1-1/p) will then be maximal | ||
+ | -} | ||
+ | primes=[2,3,5,7,11,13,17,19,23] | ||
+ | problem_69= | ||
+ | maximum [c| | ||
+ | a<-[1..length primes], | ||
+ | let b=take a primes, | ||
+ | let c=product b, | ||
+ | c<10^6 | ||
+ | ] | ||
+ | </haskell> | ||
+ | |||
+ | Note: credit for arithmetic functions is due to [http://www.polyomino.f2s.com/ David Amos]. | ||
+ | |||
+ | == [http://projecteuler.net/index.php?section=problems&id=70 Problem 70] == | ||
+ | Investigate values of n for which φ(n) is a permutation of n. | ||
+ | |||
+ | Solution: | ||
+ | <haskell> | ||
+ | import Data.List | ||
+ | isPerm a b = (show a) \\ (show b)==[] | ||
+ | flsqr n x=x<(floor.sqrt.fromInteger) n | ||
+ | pairs n1 = | ||
+ | maximum[m|a<-gena ,b<-genb,let m=a*b,n>m,isPerm m$ m-a-b+1] | ||
+ | where | ||
+ | n=fromInteger n1 | ||
+ | gena = dropWhile (flsqr n)$ takeWhile (flsqr (2*n)) primes | ||
+ | genb = dropWhile (flsqr (div n 2))$ takeWhile (flsqr n) primes | ||
+ | |||
+ | problem_70= pairs (10^7) | ||
+ | </haskell> |
Revision as of 04:58, 30 January 2008
Contents
Problem 61
Find the sum of the only set of six 4-digit figurate numbers with a cyclic property.
Solution:
import Data.List
permute [] = [[]]
permute xs = concatMap (\x -> map (x:) $ permute $ delete x xs) xs
figurates n xs = extract $ concatMap (gather (map poly xs)) $ map (:[]) $ poly n
where gather [xs] (v:vs)
= let v' = match xs v
in if v' == [] then [] else map (:v:vs) v'
gather (xs:xss) (v:vs)
= let v' = match xs v
in if v' == [] then [] else concatMap (gather xss) $ map (:v:vs) v'
match xs (_,v) = let p = (v `mod` 100)*100 in sublist (p+10,p+100) xs
sublist (s,e) = takeWhile (\(_,x) -> x<e) . dropWhile (\(_,x) -> x<s)
link ((_,x):xs) = x `mod` 100 == (snd $ last xs) `div` 100
diff (x:y:xs) = if fst x /= fst y then diff (y:xs) else False
diff [x] = True
extract = filter diff . filter link
poly m = [(n, x) | (n, x) <- zip [1..] $ takeWhile (<10000)
$ scanl (+) 1 [m-1,2*m-3..],
1010 < x, x `mod` 100 > 9]
problem_61 = sum $ map snd $ head $ concatMap (figurates 3) $ permute [4..8]
Problem 62
Find the smallest cube for which exactly five permutations of its digits are cube.
Solution:
import Data.List
import Data.Maybe
a = map (^3) [0..10000]
b = map (sort . show) a
c = (filter ((==5) . length) . group . sort) b
d = findIndex (==(head (head c))) b
problem_62 = (toInteger (fromJust d))^3
Problem 63
How many n-digit positive integers exist which are also an nth power?
Solution:
problem_63=length[x^y|x<-[1..9],y<-[1..22],y==(length$show$x^y)]
Problem 64
How many continued fractions for N ≤ 10000 have an odd period?
Solution:
import Data.List
problem_64 =length $ filter id $ map solve $ [2..9999] \\ (map (^2) [2..100])
solve n = even $ length $ cont n 0 1
cont :: Int -> Int -> Int -> [Int]
cont r n d = m : rest
where
m = truncate ((sqrt (fromIntegral r) + fromIntegral n ) / fromIntegral d)
a = n - d * m
rest = if d == 1 && n /= 0
then []
else cont r (-a) ((r - a ^ 2) `div` d)
Problem 65
Find the sum of digits in the numerator of the 100th convergent of the continued fraction for e.
Solution:
import Data.Char
import Data.Ratio
e = [2] ++ concat [ [1, 2*i, 1] | i <- [1..] ]
fraction [x] = x%1
fraction (x:xs) = x%1 + 1/(fraction xs)
problem_65 = sum $ map digitToInt $ show $ numerator $ fraction $ take 100 e
Problem 66
Investigate the Diophantine equation x2 − Dy2 = 1.
Solution:
intSqrt :: Integral a => a -> a
intSqrt n
| n < 0 = error "intSqrt: negative n"
| otherwise = f n
where
f x = if y < x then f y else x
where y = (x + (n `quot` x)) `quot` 2
problem_66 =
snd$maximum [ (x,d) |
d <- [1..1000],
let b = intSqrt d,
b*b /= d, -- d can't be a perfect square
let (x,_) = pell d b b
]
pell d wd b = piter d wd b 0 1 0 1 1 0
piter d wd b i c l k m n
| cn == 1 = (x, y)
| otherwise = piter d wd bn (i+1) cn k u n v
where
yb = (wd+b) `div` c
bn = yb*c-b
cn = (d-(bn*bn)) `div` c
yn | i == 0 = wd
| otherwise = yb
u = k*yn+l -- u/v is the i-th convergent of sqrt(d)
v = n*yn+m
(x,y) | odd (i+1) = (u*u+d*v*v, 2*u*v)
| otherwise = (u,v)
Problem 67
Using an efficient algorithm find the maximal sum in the triangle?
Solution:
problem_67 = readFile "triangle.txt" >>= print . solve . parse
parse = map (map read . words) . lines
solve = head . foldr1 step
step [] [z] = [z]
step (x:xs) (y:z:zs) = x + max y z : step xs (z:zs)
Problem 68
What is the maximum 16-digit string for a "magic" 5-gon ring?
Solution:
import Data.List
permute [] = [[]]
permute list =
concat $ map (\(x:xs) -> map (x:) (permute xs))
(take (length list)
(unfoldr (\x -> Just (x, tail x ++ [head x])) list))
problem_68 =
maximum $ map (concat . map show) poel
where
gon68 = [1..10]
knip = (length gon68) `div` 2
(is,es) = splitAt knip gon68
extnodes = map (\x -> [head es]++x) $ permute $ tail es
intnodes = map (\(p:ps) -> zipWith (\ x y -> [x]++[y])
(p:ps) (ps++[p])) $ permute is
poel = [ concat hs | hs <- [ zipWith (\x y -> [x]++y) uitsteeksels organen |
uitsteeksels <- extnodes, organen <- intnodes ],
let subsom = map (sum) hs, length (nub subsom) == 1 ]
Problem 69
Find the value of n ≤ 1,000,000 for which n/φ(n) is a maximum.
Solution:
{-phi(n) = n*(1-1/p1)*(1-1/p2)*...*(1-1/pn)
n/phi(n) = 1/(1-1/p1)*(1-1/p2)*...*(1-1/pn)
(1-1/p) will be minimal for a small p and 1/(1-1/p) will then be maximal
-}
primes=[2,3,5,7,11,13,17,19,23]
problem_69=
maximum [c|
a<-[1..length primes],
let b=take a primes,
let c=product b,
c<10^6
]
Note: credit for arithmetic functions is due to David Amos.
Problem 70
Investigate values of n for which φ(n) is a permutation of n.
Solution:
import Data.List
isPerm a b = (show a) \\ (show b)==[]
flsqr n x=x<(floor.sqrt.fromInteger) n
pairs n1 =
maximum[m|a<-gena ,b<-genb,let m=a*b,n>m,isPerm m$ m-a-b+1]
where
n=fromInteger n1
gena = dropWhile (flsqr n)$ takeWhile (flsqr (2*n)) primes
genb = dropWhile (flsqr (div n 2))$ takeWhile (flsqr n) primes
problem_70= pairs (10^7)