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module Y2017.M05.D17.Exercise where
{--
Another projecteuler.net problem? Sure.
Pentagon numbers
Problem 44
Pentagonal numbers are generated by the formula, Pn=n(3n−1)/2. The first ten
pentagonal numbers are:
1, 5, 12, 22, 35, 51, 70, 92, 117, 145, ...
It can be seen that P4 + P7 = 22 + 70 = 92 = P8. However, their difference,
70 − 22 = 48, is not pentagonal.
Find the pair of pentagonal numbers, Pj and Pk, for which their sum and
difference are pentagonal and D = |Pk − Pj| is minimised; what is the value
of D?
--}
-- gives the nth pentagon number:
pent :: Integer -> Integer
pent n = undefined
-- gives n from the pentagon number p, or nothing if p is not a pentagon number
copent :: Integer -> Maybe Integer
copent p = undefined
-- D = | Pk - Pj |, for Pentagon numbers, Pk, Pj, such that Pk + Pj is pentagon
-- and | Pk - Pj | is pentagon.
ds :: [Integer]
ds = undefined