Lucky prime

14/140927_02/140927_02.rb

@@ -0,0 +1,39 @@
+require 'prime'
+
+# Nは3以上
+N = 1123
+ary = (1..(N / 2.0).ceil).map{|i| i * 2 - 1}
+
+m = 1
+n = ary[m]
+while 2 * n <= N   # 次に取り除く数がNを超えなくなるまで続ける。
+  ary0 = []
+  ary.each_with_index{|e, i|
+    if e <=  n
+      ary0 << e
+    elsif
+      if (i + 1) % n != 0
+        ary0 << e
+      end
+    end
+  }
+  ary = ary0
+  m += 1
+  n = ary[m]
+end
+
+l_prime = []
+ary.each{|i|
+  l_prime << i if i.prime?
+}
+
+# OEIS A031157のデータ
+l_prime0 =
+[3,7,13,31,37,43,67,73,79,127,151,163,193,211,223,
+ 241,283,307,331,349,367,409,421,433,463,487,541,
+ 577,601,613,619,631,643,673,727,739,769,787,823,
+ 883,937,991,997,1009,1021,1039,1087,1093,1117,
+ 1123]
+# 一致の確認
+p l_prime == l_prime0
+