gistfile1.rb

def prime?(n)  
  (2..(n-1)).each { |x| return false if n % x==0 }  
  return true
end

def tree(number, largest_prime)
  if prime?(number)
    largest_prime = number unless number < largest_prime
    return largest_prime #Where I think the problem is
  end
	
  (2..(number-1)).each do |x|
    if number % x == 0
      largest_prime = x unless x < largest_prime
      tree(number / x, largest_prime)
    end
  end
end

tree(600851475143,0)