euler21.rb

class Fixnum
  def amicable?
    b = self.divisors.sum
    return b != self && b.divisors.sum == self
  end
  
  def divisors
    (1..self/2).select{|x| self % x == 0}
  end
end
class Array
  def sum
    self.inject(0, &:+)
  end
end
(1..10000).select(&:amicable?).sum