There are many notions of "exactness" in category theory, algebraic geometry, etc. Here I offer another that generalizes the category of frames, the notion of valuation (from probability theory), and touches on aspects of abelian categories and the Seifert-van Kampen theorem.

My intention is to hear comments from the community on

- how this notion should be refined and improved,
- other examples and fields where this notion arises,
- whether this notion fits into a larger theory or extends an existing theory.

The rough idea is that an exactness structure on a category is a set of commutative squares, such as pushout-pullbacks, and that a functor is exact when it preserves the chosen squares. In this sense, it is something like a "limit sketch".

One last note: I am aware that the term "exact square" already exists—and I'll give it as an example of what I call exact squares—so although I think the name "exact square" is fitting, I would also be happy to hear alternatives.

Let $2=\fbox{$\bullet\to\bullet$}$ denote the free arrow category, so $2\times 2$ is the free commutative square.

**Definition**: Let $C$ be a category with an initial object $\bot$. An *exactness structure* on $C$ is a set $E$ of squares, $e\colon 2\times 2\to C$, called *exact squares*
$$
\begin{array}{ccc}
A&\xrightarrow{f}&B\\
\scriptstyle g\textstyle\downarrow\;&e&\;\downarrow \scriptstyle h\\
C& \underset{i}{\to}&D
\end{array}
$$
satisfying the following conditions:

- The composite of either projection $2\times 2\to 2$ and any morphism $2\to C$ ("any degenerate square") is exact;
- The composite of the swap map $\sigma\colon 2\times 2\to 2\times 2$ and any exact square $e\colon 2\times 2\to C$ is exact;
- The pasting of any two exact squares in $C$ $$ \begin{array}{ccccc} \bullet&\to&\bullet&\to&\bullet\\ \downarrow&&\downarrow&&\downarrow\\ \bullet&\to&\bullet&\to&\bullet \end{array} $$ is exact; and
- if $e\cong e'$ are isomorphic squares then $e$ is exact iff $e'$ is.

We refer to a category with an exactness structure as an *exacting category*. We say that a functor is *exacting* if it preserves initial objects and exact squares.

We say that an exacting category $(C, \bot, E)$ is *normalized* if it has a final object and *continuous* if it has filtered colimits, and similarly morphisms are *normalized* and/or *continuous* if they preserve these structures. Let $\mathsf{ExCat}$, $\mathsf{CtsExCat}$, $\mathsf{NrmExCat}$, and $\mathsf{NrmCtsExCat}$ denote the various combinations of these adjectives.

**Example:** If $C$ is an abelian category, then it can be given the structure of a normalized exacting category. The top element is 0, and a square
$$
\begin{array}{ccc}
A&\xrightarrow{f}&B\\
\scriptstyle g\textstyle\downarrow\;&&\;\downarrow \scriptstyle h\\
C& \xrightarrow{i}&D
\end{array}
$$
is exact in the present sense iff the sequence
$$0\to A\xrightarrow{(f,g)}B\oplus C\xrightarrow{h-i}D\to 0$$
is exact in the sense of chain complexes.

**Example:** The classical Seifert-van Kampen theorem is the statement that the fundamental group functor $\pi_1\colon\mathsf{Top}\to\mathsf{Grp}$ from topological spaces to groups is exact if we choose the exact squares in $\mathsf{Top}$ to be pushout-pullback squares with simply connected pullback, and those in $\mathsf{Grp}$ to be the pushout squares.

**Example:** The category $\mathsf{Cat}$ of categories can be given the structure of a (normalized continuous) exacting category, where a square
$$
\begin{array}{ccc}
A&\xrightarrow{f}&B\\
\scriptstyle g\textstyle\downarrow\;&&\;\downarrow \scriptstyle h\\
C& \xrightarrow{i}&D
\end{array}
$$
is called exact iff it is exact in the sense of the nlab, i.e. if $g_!f^*=i^*h_!$ as functors $\mathsf{Psh}(B)\to\mathsf{Psh}(C)$.

**Example:** A frame (a.k.a. a locale), e.g. the poset of open sets in any topological space, has a continuous, normalized exactness structure. It can be regarded as a category in the usual way, its top / bottom elements serve as initial / final objects, and we say that a square is exact if it is both a pullback and a pushout:
$$
\begin{array}{ccc}
A\cap B&\to&A\\
\downarrow&&\downarrow\\
B& \to&A\cup B
\end{array}
$$
This is a full and faithful embedding $\mathsf{Frm}\to\mathsf{NrmCtsExCat}$. Indeed any monotone map between the underlying posets of frames $F$ and $F'$ that preserves top and bottom elements and filtered colimits (directed sups), is a map of frames iff it preserves binary meets and binary joins. But this is the case iff it preserves exact squares. [In fact, the functor $\mathsf{Frm}\to\mathsf{CtsExCat}$ is also fully faithful.]

**Example:** The poset $\mathbb{R}^+:=\{r\in\mathbb{R}\mid 0\leq r\}\cup\{\infty\}$ of nonnegative real numbers plus infinity under the usual $\leq$ ordering can be given a normalized continuous exactness structure where a square
$$
\begin{array}{ccc}
m&\to&n\\
\downarrow&&\downarrow\\
m'& \to&n'
\end{array}
$$
is exact iff $m+n'=m'+n$.

**Remark:** If $(C,\bot,E)$ is a (continuous) exacting category and $c\in C$ is an object, then the slice category $C_{/c}$ inherits a (continuous) exacting structure. Let $U\colon C_{/c}\to C$ be the forgetful functor. Then $C_{/c}$ inherits an initial object and filtered colimits from $C$, and we take a square $e$ to be exacting in $C_{/c}$ iff $U(e)$ is exacting in $C$.

Valuations are a constructive approach to probability theory, which agrees with the usual Kolmogorov definition in nice cases. It does not use $\sigma$-algebras but instead is defined on frames. Here we give the usual definition, except with the present terminology. Note that $\mathbb{R}^+_{/1}$ has as objects the closed interval $[0,1]$.

**Definition:** Let $F$ be a frame. A *valuation on $F$* is an exacting functor $\mu\colon F\to\mathbb{R}^+_{/1}$. It is called *normalized* and/or *continuous* if it is normalized and/or continuous as an exacting functor.

In other words, our terminology "normalized" and "continuous" was chosen to match that of valuations. The above definition situates valuations in a much broader context.

**Proposition:** Any left-exact functor preserves exacting category objects and exacting functors, normalized or not. Moreover, the direct image part of a geometric morphism preserves continuous exacting posets, such as frames and the nonnegative lower reals as described above.

Again, my question is "how will the community respond"? In other words, I'm looking for insights into this notion, how it fits with other notions I haven't discussed above, other examples of it, whether it already exists, whether there are additional requirements that should be made, etc.

Thanks!

exactif all the squares corresponding to the simplicial identities $d_id_j = d_{j-1} d_i$ (for $i < j$) are exact. Aresolutionof an object $A$ is then an exact semi-simplicial object whose colimit is $A$. $\endgroup$ – Tobias Fritz Aug 1 '18 at 2:13isomorphism, not an equality. $\endgroup$ – Mike Shulman Aug 3 '18 at 5:552more comments