Just to show a prime number of thirty one in a DRY way without any numerics.

prime_31.rb

# -*- coding: utf-8 -*-
require 'prime'

arr = []
n = Prime.first ** nil.to_i
foo = Proc.new {|x| x.times do n = Prime.first(n).last end }
bar = Proc.new {|x| arr << [[x] * x] }
Prime.first.times do |i| bar.call(Prime.first ** i) end
arr.flatten.map {|x| foo.call(x)}
puts n