Keith wrote:

> In order to do matrix multiplication, we need a multiplication and a sum operator. If we take composition {∘} to be our multiplication, what is the sum? Just simple old set union {∪}?

I intended people to just multiply and add without worrying about what it means! Physicists do this all the time - it's very useful. They call it "formal manipulations". Mathematicians call it "abstract algebra" and can't resist making the rules precise.

In other words: make up something called \\(\cdot\\) for multiplication, something called \\(+\\) for addition - and don't ask what they mean, just have them obey the usual rules of a [rig](https://en.wikipedia.org/wiki/Semiring):

* addition and multiplication are associative,

* addition is also commutative,

* multiplication distributes over addition,

* anything plus 0 is itself,

* anything times 0 is zero,

* anything times 1 is itself.

This will let you start multiplying the matrix

\[ \left( \begin{array}{cc} f & g \\\\ h & 0 \end{array} \right) \]

by itself, over and over... and you'll see all the paths in this graph start to emerge:



You'll also see why I didn't want multiplication to be commutative.

Of course you should then wonder what's going on. You might decide that \\(\cdot\\) means the same thing as composition \\(\circ\\) in our category. You might reinvent a whole bunch of math. It's more fun than learning it from a book!

Fredrick was doing something like this in the comment right before this one, but he was working less abstractly: he seems to be taking \\(\cdot\\) to be composition and \\(+\\) to be union of [multisets](https://en.wikipedia.org/wiki/Multiset), approximately speaking. But for this you need to decide what it means to compose two finite multisets of morphisms: that's where the distributive law comes in!

There's also the usual problem of whether you use \\(f\circ g\\) to mean "first \\(g\\) then \\(f\\)" or something like \\(f ;g\\) to mean "first \\(f\\) then \\(g\\)". As usual, it doesn't matter much as long as you're consistent. The second convention is a bit less annoying.