Lagrange’s Theorem is one of the two Big theorems(along with Cayley Theorem) with actual people name on it that you can find in any basic textbook of Group Theory, see how C. Pinter describes this theorem in his book AAA.

Just like Topologists can’t distinguish between a coffee cup and a doughnut(because they are homeomorphic), Group theorists have a hard time distinguishing between groups that are Isomorphic!

we will see example of two (or more) Groups, beside coming from different origin they are same in respect of Group theory.

I am omitting the basic definition of Group because I think there quite a few very good introductory content already exists, so without further ado let’s start.