for(int i=2;i<=sqrt(input);i++){
	if(input%i==0){
		return false;
	}
}
return true;

def prime(input)
  for i in 2..Math.sqrt(input) do
    return false if input % i == 0
  end
  return true
end

def prime(input)
  (2..i**0.5).all?{|i|input % i != 0}
end

require 'prime'
def prime(input)
  input.prime?
end

irb(main):005:0> require prime
NameError: undefined local variable or method 'prime' for main:Object
Did you mean?  private
	from (irb):5
	from /usr/bin/irb:11:in '<main>'

require "prime"
def prime input
  Prime.prime? input
end

Your expression caused an unhandled error:

recursive call could loop indefinitely - offset: 3