Categorification

One form of categorification takes a structure described in terms of sets, and interprets the sets as isomorphism classes of objects in a category. For example, the set of natural numbers can be seen as the set of cardinalities of finite sets (and any two sets with the same cardinality are isomorphic). In this case, operations on the set of natural numbers, such as addition and multiplication, can be seen as carrying information about coproducts and products of the category of finite sets. Less abstractly, the idea here is that manipulating sets of actual objects, and taking coproducts (combining two sets in a union) or products (building arrays of things to keep track of large numbers of them) came first. Later, the concrete structure of sets was abstracted away – taken "only up to isomorphism", to produce the abstract theory of arithmetic. This is a "decategorification" – categorification reverses this step.

Other examples include homology theories in topology. Emmy Noether gave the modern formulation of homology as the rank of certain free abelian groups by categorifying the notion of a Betti number.[3] See also Khovanov homology as a knot invariant in knot theory.

An example in finite group theory is that the ring of symmetric functions is categorified by the category of representations of the symmetric group. The decategorification map sends the Specht module indexed by partition ${\displaystyle \lambda }$ to the Schur function indexed by the same partition,

${\displaystyle S^{\lambda }{\stackrel {\varphi }{\to }}s_{\lambda },}$

essentially following the character map from a favorite basis of the associated Grothendieck group to a representation-theoretic favorite basis of the ring of symmetric functions. This map reflects how the structures are similar; for example

${\displaystyle \left[\operatorname {Ind} _{S_{m}\otimes S_{n}}^{S_{n+m}}(S^{\mu }\otimes S^{\nu })\right]\qquad {\text{ and }}\qquad s_{\mu }s_{\nu }}$

have the same decomposition numbers over their respective bases, both given by Littlewood–Richardson coefficients.

For a category ${\displaystyle {\mathcal {B}}}$, let ${\displaystyle K({\mathcal {B}})}$ be the Grothendieck group of ${\displaystyle {\mathcal {B}}}$.

Let ${\displaystyle A}$ be a ring which is free as an abelian group, and let ${\displaystyle \mathbf {a} =\{a_{i}\}_{i\in I}}$ be a basis of ${\displaystyle A}$ such that the multiplication is positive in ${\displaystyle \mathbf {a} }$, i.e.

${\displaystyle a_{i}a_{j}=\sum _{k}c_{ij}^{k}a_{k},}$ with ${\displaystyle c_{ij}^{k}\in \mathbb {Z} _{\geq 0}.}$

Let ${\displaystyle B}$ be an ${\displaystyle A}$-module. Then a (weak) abelian categorification of ${\displaystyle (A,\mathbf {a} ,B)}$ consists of an abelian category ${\displaystyle {\mathcal {B}}}$, an isomorphism ${\displaystyle \phi :K({\mathcal {B}})\to B}$, and exact endofunctors ${\displaystyle F_{i}:{\mathcal {B}}\to {\mathcal {B}}}$ such that

1. the functor ${\displaystyle F_{i}}$ lifts the action of ${\displaystyle a_{i}}$ on the module ${\displaystyle B}$, i.e. ${\displaystyle \phi [F_{i}]=a_{i}\phi }$, and
2. there are isomorphisms ${\displaystyle F_{i}F_{j}\cong \bigoplus _{k}F_{k}^{c_{ij}^{k}},}$, i.e. the composition ${\displaystyle F_{i}F_{j}}$ decomposes as the direct sum of functors ${\displaystyle F_{k}}$ in the same way that the product ${\displaystyle a_{i}a_{j}}$ decomposes as the linear combination of basis elements ${\displaystyle a_{k}}$.

• Combinatorial proof, the process of replacing number theoretic theorems by set-theoretic analogues.
• Higher category theory
• Higher-dimensional algebra
• Categorical ring

• A blog post by one of the above authors (Baez): https://golem.ph.utexas.edu/category/2008/10/what_is_categorification.html.