The pigeon hole principle !

 A very interesting subtopic of permutation and combination and enough materialistic to have a chapter of its own, we all are familiar with the Pigeonhole principle or the Dirichlet Principle.

It is a very basic principle, with a very easy statement-

If there are n number of pigeons, and (n-1) number of holes, then there has to be atleast one hole containing more than one pigeon.

It has got number number of applications, from solving very basic to super advanced level questions.

here's a copy of the notes...

https://drive.google.com/file/d/18yTuy1Kj2MQoihb2Nhipac_3y_JOMP7f/view?usp=sharing