A Mathematician, a dog and a cow

A mathematician walks into a bar with his dog and cow in tow

“Hey, buddy," the bartender says, "no animals are allowed in here!”

“These are very special animals," replies the mathematician.

“Yeah?  How so?”

“They’re knot theorists.”

The bartender sneers and says, “I’ve met a bunch of knot theorists who I thought were animals, but no an animals that were knot theorists.”

The mathematician shrugs and says "I can prove it.  Ask them anything you like."

The bartender turns to the dog and asks, “Name a knot invariant.” “Arf!” the dog barks.

Turning to the cow, the bartender asks “Name a topological invariant.” “Mu!” says the cow.

"Very funny.  Get out!" the bartender demands.

Sitting on the curb outside, the dog turns to the mathematician and asks, “You think I should have said the Jones polynomial instead?”