# Difference between revisions of "Euler problems/41 to 50"

(→[http://projecteuler.net/index.php?section=problems&id=46 Problem 46]: solution inspired by SICP exercise 3.70) |
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Line 43: | Line 43: | ||

Solution: | Solution: | ||

+ | |||

+ | This solution is inspired by exercise 3.70 in ''Structure and Interpretation of Computer Programs'', (2nd ed.). | ||

+ | |||

<haskell> | <haskell> | ||

− | problem_46 = | + | problem_46 = head $ oddComposites `orderedDiff` gbSums |

+ | |||

+ | oddComposites = filter ((>1) . length . primeFactors) [3,5..] | ||

+ | |||

+ | gbSums = map gbWeight $ weightedPairs gbWeight primes [2*n*n | n <- [1..]] | ||

+ | gbWeight (a,b) = a + b | ||

+ | |||

+ | weightedPairs w (x:xs) (y:ys) = | ||

+ | (x,y) : mergeWeighted w (map ((,)x) ys) (weightedPairs w xs (y:ys)) | ||

+ | |||

+ | mergeWeighted w (x:xs) (y:ys) | ||

+ | | w x <= w y = x : mergeWeighted w xs (y:ys) | ||

+ | | otherwise = y : mergeWeighted w (x:xs) ys | ||

+ | |||

+ | x `orderedDiff` [] = x | ||

+ | [] `orderedDiff` y = [] | ||

+ | (x:xs) `orderedDiff` (y:ys) | ||

+ | | x < y = x : xs `orderedDiff` (y:ys) | ||

+ | | x > y = (x:xs) `orderedDiff` ys | ||

+ | | otherwise = xs `orderedDiff` ys | ||

</haskell> | </haskell> | ||

## Revision as of 05:02, 28 March 2007

## Contents

## Problem 41

What is the largest n-digit pandigital prime that exists?

Solution:

```
problem_41 = undefined
```

## Problem 42

How many triangle words can you make using the list of common English words?

Solution:

```
problem_42 = undefined
```

## Problem 43

Find the sum of all pandigital numbers with an unusual sub-string divisibility property.

Solution:

```
problem_43 = undefined
```

## Problem 44

Find the smallest pair of pentagonal numbers whose sum and difference is pentagonal.

Solution:

```
problem_44 = undefined
```

## Problem 45

After 40755, what is the next triangle number that is also pentagonal and hexagonal?

Solution:

```
problem_45 = undefined
```

## Problem 46

What is the smallest odd composite that cannot be written as the sum of a prime and twice a square?

Solution:

This solution is inspired by exercise 3.70 in *Structure and Interpretation of Computer Programs*, (2nd ed.).

```
problem_46 = head $ oddComposites `orderedDiff` gbSums
oddComposites = filter ((>1) . length . primeFactors) [3,5..]
gbSums = map gbWeight $ weightedPairs gbWeight primes [2*n*n | n <- [1..]]
gbWeight (a,b) = a + b
weightedPairs w (x:xs) (y:ys) =
(x,y) : mergeWeighted w (map ((,)x) ys) (weightedPairs w xs (y:ys))
mergeWeighted w (x:xs) (y:ys)
| w x <= w y = x : mergeWeighted w xs (y:ys)
| otherwise = y : mergeWeighted w (x:xs) ys
x `orderedDiff` [] = x
[] `orderedDiff` y = []
(x:xs) `orderedDiff` (y:ys)
| x < y = x : xs `orderedDiff` (y:ys)
| x > y = (x:xs) `orderedDiff` ys
| otherwise = xs `orderedDiff` ys
```

## Problem 47

Find the first four consecutive integers to have four distinct primes factors.

Solution:

```
problem_47 = undefined
```

## Problem 48

Find the last ten digits of 11 + 22 + ... + 10001000.

Solution:

```
problem_48 = undefined
```

## Problem 49

Find arithmetic sequences, made of prime terms, whose four digits are permutations of each other.

Solution:

```
problem_49 = undefined
```

## Problem 50

Which prime, below one-million, can be written as the sum of the most consecutive primes?

Solution:

```
problem_50 = undefined
```