Q:

Ruby Rails, global function available to all controllers

This is probably a stupid question, can you have a function available to multiple controllers? 
I have 2 controllers
class C1Controller < ActionController::Base
 def add(input_value)
  @output_value = input_value * 2
 end
end

now I would like to use this add function in another controller like so
class C2Controller < ActionController::Base
 @new_value = add(2)
end

A:

Define that function in app/controllers/application_controller.rb, and it will be available in all controllers.