150806

Ruby


Right-truncatable prime

オンライン整数列大辞典の
A024770（http://oeis.org/A024770/list）
と比較し、答え合わせしてみる。

require 'prime'

N = 49

p_ary = [2, 3, 5, 7]

odds = [1, 3, 5, 7, 9]

ary = p_ary.clone

while p_ary.size > 0

  new_p_ary = []

  p_ary.each{|i|

    odds.each{|j|

      k = (i.to_s + j.to_s).to_i

      if k.prime?

        new_p_ary.push(k)

        ary.push(k)

      end

    }

  }

  p_ary = new_p_ary

end

# 全部表示

p ary

# N番目まで

ary = ary[0..N - 1]

# OEIS A024770のデータ

ary0 =

[2,3,5,7,23,29,31,37,53,59,71,73,79,233,239,293,

 311,313,317,373,379,593,599,719,733,739,797,2333,

 2339,2393,2399,2939,3119,3137,3733,3739,3793,3797,

 5939,7193,7331,7333,7393,23333,23339,23399,23993,

 29399,31193]

# 一致の確認

p ary == ary0

出力結果

[2, 3, 5, 7, 23, 29, 31, 37, 53, 59, 71, 73, 79, 233, 239, 293, 311, 313, 317, 3

73, 379, 593, 599, 719, 733, 739, 797, 2333, 2339, 2393, 2399, 2939, 3119, 3137,

 3733, 3739, 3793, 3797, 5939, 7193, 7331, 7333, 7393, 23333, 23339, 23399, 2399

3, 29399, 31193, 31379, 37337, 37339, 37397, 59393, 59399, 71933, 73331, 73939,

233993, 239933, 293999, 373379, 373393, 593933, 593993, 719333, 739391, 739393,

739397, 739399, 2339933, 2399333, 2939999, 3733799, 5939333, 7393913, 7393931, 7

393933, 23399339, 29399999, 37337999, 59393339, 73939133]

true