nLab
factorization system over a subcategory

Idea

A factorization system over a subcategory is a common generalization of an orthogonal factorization system and a strict factorization system, in which factorizations are only unique up to zigzags belonging to some specified subcategory.

Definition

Let $C$ be a subcategory, and let $J$, $E$, and $M$ be wide subcategories of $C$ with $J\subseteq E$ and $J\subseteq M$. Given a morphism $f:x\to y$ in $C$, let ${\mathrm{Fact}}_J^{E,M}(f)$ denote the non-full subcategory of the over-under-category (double comma category) $(x/C/y)$:

• whose objects are pairs $x\to z\to y$ such that $x\to z$ is in $E$, $z\to y$ is in $M$, and the composite $x\to y$ is $f$;
• whose morphisms from $x\to z\to y$ to $x\to z\prime \to y$ are morphisms $z\to z\prime$ which are in $J$ and make the two evident triangles commute.

We say that $(E,M)$ is a factorization system over $J$ if ${\mathrm{Fact}}_J^{E,M}(f)$ is connected (and thus, in particular, inhabited).

Examples

• If $J$ consists of only the identities in $C$, then a factorization system over $J$ is a strict factorization system.

• If $J$ is the core of $C$, then a factorization system over $J$ is an orthogonal factorization system

• If $J$ is the canonical inclusion of (a skeleton of) ${\mathrm{FinSet}}^{\mathrm{op}}$ into a Lawvere theory $C$, then a factorization system over $J$ is a decomposition of $C$ into a distributive law of two other Lawvere theories.

Relation to distributive laws

Suppose given a category $J$. Then to give a category $C$ equipped with an identity-on-objects functor $J\to C$ and a factorization system over $J$ is the same as to give a distributive law between two monads on $J$ in the bicategory Prof. The two monads are the categories $E$ and $M$, and their composite is $C$.

References

• Eugenia Cheng, Distributive laws for Lawvere theories, arXiv:1112.3076