perfects.rb

class PerfectNumbers
  attr_reader :perfects
  def initialize(len = nil)
    return nil if len.nil?
    
    # sequence A000043 in OEIS
    # http://www.research.att.com/~njas/sequences/A000043
    n = [2, 3, 5, 7, 13, 17, 19, 31, 61, 89, 107, 127, 521, 607, 1279, 2203, 
      2281, 3217, 4253, 4423, 9689, 9941, 11213, 19937, 21701, 23209, 44497, 
      86243, 110503, 132049, 216091, 756839, 859433, 1257787, 1398269, 
      2976221, 3021377, 6972593, 13466917]
    
    @perfects = Array.new
    (0..len.abs-1).each do
      |i|
      @perfects << (2**(n[i]-1)) * (2**n[i] - 1)
    end
  end
end

run.rb

require 'perfects.rb'

p = PerfectNumbers.new(10)
puts p.perfects