MY_Controller not found

Hi all, sorry if I bother with stupid question...
I try to build a class with some basic function (login etc), I want to put as core class and include that on controller that need that functions.
For example

I have a controller Pippo.php in Controllers folder it exend MY_Controller that is located in core folder...
Well I can't find a way to let this working...wherever I try I get the error the MY_Controller is not found...
How can I solve this =( in config the MY_ prefix is already set

This message could be if the class name in MY_Controller.php is not MY_Controller.

Code:

class MY_Controller extends CI_Controller {

}

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