160706

Ruby

Numbers n such that 2 * n^2 - 1 is a square(1)

オンライン整数列大辞典の
A001653（http://oeis.org/A001653/list）
と比較し、答え合わせしてみる。

def A001653(n)
  a, b, c = 1, 5, 29
  ary = [a, b, c]
  while ary.size < n
    a, b, c = b, c, 6 * c - b
    ary << c
  end
  ary[0..n - 1]
end
ary = A001653(23)

# OEIS A001653のデータ
ary0 =
[1,5,29,169,985,5741,33461,195025,1136689,6625109,
 38613965,225058681,1311738121,7645370045,
 44560482149,259717522849,1513744654945,
 8822750406821,51422757785981,299713796309065,
 1746860020068409,10181446324101389,
 59341817924539925]
# 一致の確認
p ary == ary0