Usage: 10.phi #=> 4

phi.rb

class Integer
  def phi  
    fractions = self.prime_division
    primes = fractions.map { |b,e| b }
    rationals = primes.map {|p| Rational(p-1,p) }
    return (rationals.reduce { |a,b| a*b } * self).to_i
  end  
end