## Category Theory Product

Before we discuss this week's topic, I want to make two remarks from the prior posts content. First, why we need natural transformations in algebraic topology? Associating groups to topological spaces (which incidentally describe the hole structure of the space) is done by the use of functors. Different (co)homology theories are basically different functors, and their equivalence is the same as proving the existence of a natural transformation. Second, the logic used in category theory is intuitionistic logic where truth is proved constructively. Since this is mapped into computer science by the Curry-Howard isomorphism, the fact that some statements have no constructive proof is equivalent with a computation running forever. In computation theory one encounters the halting problem. If the halting problem were decidable then category theory would have been mapped to ordinary logic instead of intuitionistic logic.

Now back to the topic of the day. We are still in pure math domain and we are looking at mathematical objects from 10,000 feet disregarding their nature and observing only their existence and their relationships (objects and arrows). The first question one asks is how to construct new categories from existing ones? One way is to simply reverse the direction of all arrows and the resulting category is unsurprisingly called the opposite category (or the dual). Another way is to combine two category into a new one. Enter the concept of a product of two categories: $$\mathbf{C}\times \mathbf{D}$$. In set theory this would correspond with to the Cartesian product of two sets. However we need to give a definition which is independent of the nature of the elements. Moreover we want to give it in a way which guarantees uniqueness up to isomorphism.

The basic idea is that of a projection from the elements of $$\mathbf{C}\times \mathbf{D}$$ back to the elements of $$\mathbf{C}$$ and $$\mathbf{D}$$. So how do we know that those projections and the product is unique up to isomorphism? Suppose that there is another category $$\mathbf{Y}$$ with maps $$f_C$$ and $$f_D$$. Then there is a unique map $$f$$ such that the diagram below commutes

This diagram has to commute for all categories $$\mathbf{Y}$$ and their maps $$f_C$$ and $$f_D$$. From this definition, can you prove uniqueness of the product up to isomorphism? It is a simple matter of "diagram reasoning". Just pretend that Y is now the "true incarnation" of the product. You need to find a morphisms f from Y to CxD and a morphism g from CxD to Y such that $$g\circ f =1_{C\times D}$$, $$g\circ f = 1_Y$$. See? Category theory is really easy and not harder than linear algebra.

Now what happens if we flip all arrows in the diagram above? We obtain a coproduct category $$\mathbf{C}\oplus \mathbf{D}$$ and the projections maps become injection maps.

OK, time for concrete examples:

• sets: product = Cartesian product, coproduct = disjoint union
• partial order sets: product = greatest lower bounds (meets), coproduct = least upper bounds (joins)
So where are we now? The concept of the product is very simple, but we need it as a stepping stone to the concept of tensor product and (symmetric) monoidal category. Why? Because physical systems form a symmetric monoidal category. Using categorical arguments we can derive the complete mathematical properties of any theory of nature describing a symmetric monoidal category. And the answer will turn out to be: quantum mechanics. Please stay tuned.