recursive_prime_factors.rb

def prime_factors(number)
  factors = []
  2.upto(number/2) do |possible_divisor|
    if number % possible_divisor == 0
       factors << possible_divisor
       prime_factors(number /= possible_divisor) 
    end  
  end
  if number > 1
    factors << number 
  end
  factors
end