class Integer

def prime?

Returns true if +self+ is a prime number, else returns false.
def prime?
  return self >= 2 if self <= 3
  return false if self % 2 == 0 or self % 3 == 0
  (5..(self**0.5).floor).step(6).each do |i|
    if self % i == 0 || self % (i + 2) == 0
      return false
    end
  end
  true
end