Project Euler Problem 44

問題

Pentagonal numbers are generated by the formula, P_(n)=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 P(4) + P(7) = 22 + 70 = 92 = P(8). However, their difference,
70 − 22 = 48, is not pentagonal.

Find the pair of pentagonal numbers, P(j) and P(k), for which their sum and difference
is pentagonal and D = |P(k) − P(j)| is minimised; what is the value of D?

ソース

max = 10000
i2p = [0] + (1..max).map{|i| i * (i * 3 - 1) / 2}
p2i = {}
i2p[1..max].each_index{|i|p2i[i2p[i]] = i}

min = i2p.last
(1...max).each{|j|
	(j + 1..max).each{|k|
		next if p2i[i2p[k] + i2p[j]].nil?
		next if p2i[i2p[k] - i2p[j]].nil?
		puts "#{i2p[k]}, #{i2p[j]}"
		min = [min, i2p[k] - i2p[j]].min
	}
}
puts min

解答

5482660

感想

そんなんすぐに現れるだろうと思って、五角数のmaxをもっと少なくしていました。。。
5482660って結構ないんですね（汗