### This Week’s Finds in Mathematical Physics (Week 249)

#### Posted by John Baez

In week249 of This Week’s Finds you can finally see Felix Klein’s famous "Erlangen program" for reducing geometry to group theory - translated into English!

Then, continue reading the Tale of Groupoidification - in which we see how group actions are just groupoids equipped with extra stuff.

Posted at April 9, 2007 4:15 AM UTC
## Re: This Week’s Finds in Mathematical Physics (Week 249)

Thanks John! Fascinating as always:

These are exactly the things which crop up when you consider morphisms between 2-representations of $G$. Can’t wait for next time.