nLab Beck-Chevalley condition

Idea

The Beck–Chevalley condition, also sometimes called just the Beck condition or the Chevalley condition, is a “commutation of adjoints” property that holds in many “change of base” situations.

Motivation

The Beck-Chevalley condition may be understood as a natural compatibility condition for

(1) integral transforms in geometry

(2) quantifiers in formal logic/type theory

From integral transforms

From the point of view of geometry, in contexts such as of integral transforms one considers correspondences between spaces $A$, $B$ given by spans of maps between them:

Via composition of such correspondences by fiber product of adjacent legs, they form the 1-morphisms in a 2-category Span.

Assuming some base change adjoint pair $f^\ast \vdash f_\ast$, thought of as

• push-forward $f_\ast \,\colon\, \mathcal{D}(X) \longrightarrow \mathcal{D}(A)$

and

• pull-back $f^\ast \,\colon\, \mathcal{D}(A) \longrightarrow \mathcal{D}(X)$

is functorially associated with maps $f$ (i.e. such that there are natural isomorphisms $(f_2 f_1)_\ast \,\simeq\, (f_2)_\ast (f_1)_\ast$ and dually), the integral transform encoded by any span as above is the “pull-push” operation given by $g_* f^*$.

Now the Beck-Chevalley condition (on such assignment of base change adjoints to maps) essentially says that this pull-push construction is (2-)functorial under composition of spans:

Concretely, given a pair of composable spans:

and their composite in Span formed by the fiber product of the adjacent legs

then functoriality of pull-push means that the two different ways to pull-push through the diagram coincide:

Pull-pushing through the spans separately results in

$$(k_\ast h^\ast) \circ (g_\ast f^\ast) \;=\; k_\ast \, \underbrace{h^\ast \, g_\ast} \, f^\ast \,,$$

while pull-pushing through the composite span results in

$$(k p)_* (f q)^* = k_* \, \underbrace{p_* \, q^*} \, f^* \,.$$

For these two operations to coincide, up to isomorphism, hence for pull-push to respect composition of spans, we need an isomorphism between the operations shown over braces, hence of the form

(1)$$h^\ast \, g_\ast \;\simeq\; p_* \, q^*$$

between the two ways of push-pulling along the sides of any pullback square

This (1) is the Beck-Chevalley condition: A natural necessary condition to ensure that integral transforms by pull-push base change through correspondences is functorial under composition of correspondences by fiber product of adjacent legs.

More motivation along these lines also be found at dependent linear type theory. As a formulation of propagation in cohomological quantum field theory this is Sc14 there.

From logic and type theory

From the point of view of formal logic and dependent type theory, the three items $f_! \dashv f^\ast \vdash f_\ast$ in a base change adjoint triple constitute the categorical semantics of quantification and context extension (existential quantification/dependent pair type $\dashv$ context extension $\dashv$ universal quantification/dependent product type), and so the Beck-Chevalley condition says that these are compatible with each other: concretely that substitution of free variables commutes with quantification — a condition which syntactically is “self-evident”, at least it is evidently desirable.

For more on this logical aspect see below.

Definition

Suppose given a commutative square (up to isomorphism) of functors:

$$\array{ & \overset{f^*}{\to} & \\ ^{g^*}\downarrow && \downarrow^{k^*}\\ & \underset{h^*}{\to} & }$$

in which $f^*$ and $h^*$ have left adjoints $f_!$ and $h_!$, respectively. (The classical example is a Wirthmüller context.) Then the natural isomorphism that makes the square commute

$$k^* f^* \to h^* g^*$$

has a mate

$$h_! k^* \to g^* f_!$$

defined as the composite

$$h_! k^* \overset{\eta}{\to} h_! k^* f^* f_! \overset{\cong}{\to} h_! h^* g^* f_! \overset{\epsilon}{\to} g^* f_! \,.$$

We say the original square satisfies the Beck–Chevalley condition if this mate is an isomorphism.

More generally, it is clear that for this to make sense, we only need a transformation $k^* f^* \to h^* g^*$; it doesn’t need to be an isomorphism. We also use the term Beck–Chevalley condition in this case,

Left and right Beck–Chevalley condition

Of course, if $g^*$ and $k^*$ also have left adjoints, there is also a Beck–Chevalley condition stating that the corresponding mate $k_! h^* \to f^* g_!$ is an isomorphism, and this is not equivalent in general. Context is usually sufficient to disambiguate, although some people speak of the “left” and “right” Beck–Chevalley conditions.

Note that if $k^* f^* \to h^* g^*$ is not an isomorphism, then there is only one possible Beck-Chevalley condition.

Dual Beck–Chevalley condition

If $f^*$ and $h^*$ have right adjoints $f_*$ and $h_*$, there is also a dual Beck–Chevalley condition saying that the mate $g^* f_* \to h_* k^*$ is an isomorphism. By adjointness, if $f^*$ and $h^*$ have right adjoints and $g^*$ and $k^*$ have left adjoints, then $g^* f_* \to h_* k^*$ is an isomorphism if and only if $k_! h^* \to f^* g_!$ is.

For bifibrations

Originally, the Beck-Chevalley condition was introduced in (Bénabou-Roubaud, 1970) for bifibrations over a base category with pullbacks. In loc.cit. they call this condition Chevalley condition because he introduced it in his 1964 seminar.

A bifibration $\mathbf{X} \to \mathbf{B}$ where $\mathbf{B}$ has pullbacks satisfies the Chevalley condition iff for every commuting square

$$\array{ & \overset{\psi^\prime}{\rightarrow} & \\ \downarrow^{\varphi^\prime} && \downarrow^{\varphi}\\ & \underset{\psi}{\rightarrow} & }$$

in $\mathbf{X}$ over a pullback square in the base $\mathbf{B}$ where $\varphi$ is cartesian and $\psi$ is cocartesian it holds that $\varphi^\prime$ is cartesian iff $\psi^\prime$ is cocartesian. Actually, it suffices to postulate one direction because the other one follows. The nice thing about this formulation is that there is no mention of “canonical” morphisms and no mention of cleavages.

A fibration $P$ has products satisfying the Chevalley condition iff the opposite fibration $P^{op}$ is a bifibration satisfying the Chevalley condition in the above sense.

According to the Benabou–Roubaud theorem, the Chevalley condition is crucial for establishing the connection between the descent in the sense of fibered categories and the monadic descent.

“Local” Beck–Chevalley condition

Suppose that $f^*$ and $h^*$ do not have entire left adjoints, but that for a particular object $x$ the left adjoint $f_!(x)$ exists. This means that we have an object “$f_! x$” and a morphism $\eta_x\colon x \to f^* f_! x$ which is initial in the comma category $(x / f^*)$. Then we have $k^*(\eta) \colon k^* x \to k^* f^* f_! x \to h^* g^* f_! x$, and we say that the square satisfies the local Beck-Chevalley condition at $x$ if $k^*(\eta)$ is initial in the comma category $(k^* x / h^*)$, and hence exhibits $g^* f_! x$ as “$h_! k^* x$” (although we have not assumed that the entire functor $h_!$ exists).

If the functors $f_!$ and $h_!$ do exist, then the square satisfies the (global) Beck-Chevalley condition as defined above if and only if it satisfies the local one defined here at every object.

In logic / type theory

If the functors in the formulation of the Beck-Chevalley condition are base change functors in the categorical semantics of some dependent type theory (or just of a hyperdoctrine) then the BC condition is equivalently stated in terms of logic as follows.

A commuting diagram

$$\array{ D &\stackrel{h}{\to}& C \\ {}^{\mathllap{k}}\downarrow && \downarrow^{\mathrlap{g}} \\ A &\stackrel{f}{\to}& B }$$

is interpreted as a morphism of contexts. The pullback (of slice categories or of fibers in a hyperdoctrine) $h^*$ and $f^*$ is interpreted as the substitution of variables in these contexts. And the left adjoint $\sum_k \coloneqq k_!$ and $\sum_q \coloneqq g_!$, the dependent sum is interpreted (up to (-1)-truncation, possibly) as existential quantification.

In terms of this the Beck-Chevalley condition says that if the above diagram is a pullback, then substitution commutes with existential quantification

$$\sum_k h^* \phi \stackrel{\simeq}{\to} f^* \sum_g \phi \,.$$

Example

Consider the diagram of contexts

$$\array{ [\Gamma, x : X] &\stackrel{}{\to}& [\Gamma, x : X, y : Y] \\ \downarrow && \downarrow \\ \Gamma &\to& [\Gamma, y : Y] } \;\;\; \simeq \;\;\; \array{ \Gamma \times X &\stackrel{(p_1,p_2,t)}{\to}& \Gamma \times X \times Y \\ {}^{\mathllap{p_1}}\downarrow && \downarrow^{\mathrlap{(p_1,p_3)}} \\ \Gamma &\stackrel{(id,t)}{\to} & \Gamma \times Y } \,,$$

with the horizontal morphism coming from a term $t : \Gamma \to Y$ in context $\Gamma$ and the vertical morphisms being the evident projection, then the condition says that we may in a proposition $\phi$ substitute $t$ for $y$ before or after quantifying over $x$:

$$\sum_{x : X} \phi(x,t) \simeq (\sum_{x : X} \phi(x,y))[t/y] \,.$$

Examples

Various

• The codomain fibration of any category with pullbacks is a bifibration, and satisfies the Beck–Chevalley condition at every pullback square.

• If $C$ is a regular category (such as a topos), the bifibration $Sub(C) \to C$ of subobjects satisfies the Beck–Chevalley condition at every pullback square.

• The family fibration $Fam(C)\to Set$ of any category $C$ with small sums satisfies the Beck–Chevalley condition at every pullback square in $Set$.

• Mackey’s restriction formula for group representations.

For categories of presheaves

Proposition

If $\phi : D \to C$ is an opfibration of small categories and

$$\array{ D' &\stackrel{\beta}{\to}& D \\ \downarrow^{\mathrlap{\psi}} && \downarrow^{\mathrlap{\phi}} \\ C' &\stackrel{\alpha}{\to}& C }$$

is a pullback diagram (in the 1-category Cat), and for $\mathcal{C}$ a category with all small colimits, then the induced diagram of presheaf categories

$$\array{ [D', \mathcal{C}] &\stackrel{\beta^*}{\longleftarrow}& [D, \mathcal{C}] \\ \uparrow^{\mathrlap{\psi}^*} && \uparrow^{\mathrlap{\phi}^*} \\ [C', \mathcal{C}] &\stackrel{\alpha^*}{\longleftarrow}& [C,\mathcal{C}] } \,,$$

satisfies the Beck-Chevalley condition: for $\psi_!$ and $\phi_!$ denoting the left Kan extension along $\psi$ and $\phi$, respectively, then we have a natural isomorphism

$$\psi_! \beta^* \simeq \alpha^* \phi_! \,.$$

(This is maybe sometimes called the projection formula. But see at projection formula.)

For this statement in the more general context of quasicategories see Joyal 2008, prop. 11.6.

Proof

Since $\phi$ is opfibered, for every object $c \in C$ the embedding of the fiber $\phi^{-1}(c)$ into the comma category $\phi/c$ is a final functor. Therefore the pointwise formula for the left Kan extension $\phi_!$ is equivalently given by taking the colimit over the fiber, instead of the comma category

$$\phi_1(X)_c \simeq \lim_{\underset{\phi^{-1}(c)}{\to}} X \,.$$

Therefore we have a sequence of isomorphisms

$$\begin{aligned} (\psi_! \beta^* X)(c') & \simeq \lim_{\underset{\psi^{-1}(c')}{\to}} (X\circ \beta) \\ & \simeq \lim_{\underset{\phi^{-1}(\alpha(c'))}{\to}} X \\ & \simeq (\alpha^* \phi_! X)(c') \end{aligned}$$

all of them natural in $c'$.

For an example that prop. may fail without the condition that $D \to C$ is an opfibration:

Consider $C=$ the interval category $(0\to 1)$, $D=C'=$ the terminal category, $\phi=0$, $\alpha=1$, so that $D'=\emptyset$, but $\alpha^*\phi_!$ is the identity functor.

Proper base change in étale cohomology

For coefficients of torsion group, étale cohomology satisfies Beck-Chevalley along proper morphisms. This is the statement of the proper base change theorem. See there for more details.

Grothendieck six operations

A kind of Beck-Chevalley condition appears in the axioms of Grothendieck’s six operations. See there for more.

Related pages

• exact square
• Benabou-Roubaud theorem

References

The Beck-Chevalley condition has arisen in the theory of descent - as developed from Grothendieck 1959. Jon Beck and Claude Chevalley studied it independently from each another. […] It is conspicuous that neither of them ever published anything on it. [Pavlović 1991, §14]

Original articles:

• Jean Bénabou, Jacques Roubaud, Monades et descente, C. R. Acad. Sc. Paris, Ser. A 270 (1970) 96-98 [gallica:12148/bpt6k480298g/f100, pdf, English translation (by Peter Heinig): MO:q/279152]

  (proving the Bénabou-Roubaud theorem)

• William Lawvere, p. 8 of: Equality in hyperdoctrines and comprehension schema as an adjoint functor, Proceedings of the AMS Symposium on Pure Mathematics XVII (1970) 1-14 [pdf]

  (in terms of base change as the categorical semantics for universal quantification/dependent product and existential quantification/dependent sum)

Review:

• Duško Pavlović, Section 1 of: Categorical interpolation: Descent and the Beck-Chevalley condition without direct images, in: Category Theory, Lecture Notes in Mathematics 1488 (1991) [doi:10.1007/BFb0084229, pdf]

• Paul Balmer, around §7.5 of: Stacks of group representations, J. European Math. Soc. 17 1 (2015) 189-228 [arXiv:1302.6290, doi:10.4171/jems/501]

and in the generality of $\infty$-categories:

• Urs Schreiber, around Def. 5.5 of: Quantization via Linear homotopy types [arXiv:1402.7041]

• Dennis Gaitsgory, Jacob Lurie, Section 2.4.1 of: Weil’s conjecture for function fields (2014-2017) [“first volume of expanded account” pdf]

Discussion for subobject lattices:

• Saunders MacLane, Ieke Moerdijk, chapter IV.9 (around p. 205) of: Sheaves in Geometry and Logic, Springer (1992) [doi:10.1007/978-1-4612-0927-0]

Discussion for presheaf categories in the context of quasicategories ($\infty$-categories of $\infty$-presheaves):

• André Joyal, p. 175 of: The Theory of Quasi-Categories and its Applications, lectures at Advanced Course on Simplicial Methods in Higher Categories, CRM (2008) [pdf, pdf]

Discussion for Goursat categories:

• Marino Gran, Diana Rodelo, Beck-Chevalley condition and Goursat categories, Journal of Pure and Applied Algebra 221 10 (2017) 2445-2457 [arXiv:1512.04066, doi:10.1016/j.jpaa.2016.12.031]