MathOverflow is a question and answer site for professional mathematicians. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

You can present a group in a Cayley-like manner, replacing colors by explicit assignment of nodes to edges: while in a Cayley graph $x \circ y = z$ is presented like this:

alt text

you can also present it like this:

alt text

Now the group axioms can be stated like this:

  • For each $x, y$ there is a unique $z$ with $x \circ y = z$, or: such that the following diagram holds ("commutes"):

alt text

  • There is an $e$ such that for all $x$ it holds that $x\circ e = e \circ x = x$, or: such that the following two diagrams commute:

alt text

and for each $x$ there is a $x^{-1}$ such that $x \circ x^{-1} = x^{-1} \circ x = e$, or: such that the following diagram commutes:

alt text

  • For each $x, y, z$ it holds that $x \circ (y \circ z) = (x \circ y) \circ z$, or: such that the following diagram commutes:

alt text

The last diagram is somewhat ugly, even when drawn in this most balanced way (I didn't find a more appealing and symmetric one).

But an astonishing symmetry arises, when we consider Abelian groups. Commutativity is expressed by the diagram:

alt text

and associativity becomes:

alt text

In the presence of commutativity, associativity seems to be related to commutativity (some sort of "second level commutativity").

Can any use be made of this kind of diagrams, or is it just vain baublery?

share|cite|improve this question
I guess your diagram for associativity in general groups is just a kind of ugly, because you draw it in two dimensions. In three dimensions you can put the squares as corners of tetrahedron and the circles on the edges ;-). (OK, the directions of the arrows are not so clear.) – Someone Feb 16 '11 at 17:51
It looks a bit like "sketches" to me, but I'm not an expert so shall merely mention it. As for your comment about commutativity and associativity, associativity is commutativity. It's commutativity of the operations $L_x$ and $R_z$ where $L_x$ is "multiply on the left by $x$" and $R_z$ is (you've guessed it), "multiply on the right by $z$". I have a vague memory of seeing an article going into some detail about this. Under commutativity, $R_z = L_z$ so we can rewrite $x(y z)$ as $x (z y)$ and $(x y) z$ as $z (x y)$ whereupon associativity becomes commutativity of $L_x$ and $L_z$. – Loop Space Feb 16 '11 at 18:23
I vote for baublery. – Martin Brandenburg Feb 17 '11 at 0:12

These pictures are a way of writing a group as an algebra over an operad. The little square is the 2-ary operation, and the arrows are an indicator that makes the inputs distinguishable.

John Baez has written about operads using pictures similar to yours. See for example TWF week 191.

The relation between associativity and commutativity is similar to a fact seen in some books on vertex algebras, where one starts with an axiom like $x(yz) = y(xz)$ and after some power series manipulations deduces $x(yz) = (xy)z$.

share|cite|improve this answer

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.