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A Wirthmüller context is a pair of two symmetric closed monoidal categories $(\mathcal{X}, \otimes_X, 1_{X})$, $(\mathcal{Y}, \otimes_Y, 1_Y)$ which are connected by an adjoint triple of functors such that the middle one is a closed monoidal functor.
This is the variant/special case of the yoga of six operations consisting of two adjoint pairs $(f_! \dashv f^!)$ and $(f^\ast \dashv f_\ast)$ and the tensor product/internal hom adjunctions $((-)\otimes B \dashv [B,-])$, specialized to the case that $f^! \simeq f^\ast$:
(The other specialization of the six operations to $f_\ast \simeq f_!$ is called the Grothendieck context).
Often one is interested in the case that there is an object $C \in \mathcal{X}$ and an equivalence
If this induces a natural equivalence
for $A \in \mathcal{X}$, then one says this is a Wirthmüller isomorphism, following (Wirthmueller 74). In particular there is a canonical natural transformation
and one can ask this to be an equivalence, hence a Wirthmüller isomorphism (May 05).
Let $(\mathcal{X}, \otimes_X, 1_{X})$, $(\mathcal{Y}, \otimes_Y, 1_Y)$ be two symmetric closed monoidal categories and let
be an adjoint triple of functors between them. We call this setup
a pre-Wirthmüller context if $f^\ast$ is a strong monoidal functor:
a Wirthmüller context if $f^\ast$ is in addition a strong closed functor, hence a strongly closed monoidal functor.
We write $[-,-]$ for the internal hom functors. For $A \in \mathcal{X}$ we write
for the internal hom from $A$ into the unit object, hence for dual of $A$ with respect to the closed category structure, its dual object in a closed category.
We say “$A$ is dualizable” to mean that it is a dualizable object with respect, instead, to the (symmetric) monoidal category structure $\otimes_X$. If $A$ is dualizable we write $A^\vee$ for its monoidal dual object.
Similarly for $B \in \mathcal{Y}$.
If all objects in $\mathcal{X}$ and $\mathcal{Y}$ are dualizable, hence if they are compact closed categories, then they are in particular also star-autonomous categories with dualizing object the tensor unit. As such their internal logic is linear logic and their type theory is linear type theory. In terms of this a Wirthmüller morphism, def. , is the linear analog of a context extension morphism in a hyperdoctrine: it defines a dependent linear type theory. See there for more.
In a pre-Wirthmüller context, def. , there is a canonical natural transformation
(not necessarily an equivalence) being the adjunct of the composite
where the first morphism is the tensor product of two copies of the adjunction unit and where the second is the equivalence that exhibits $f^\ast$ as a strong monoidal functor.
In a pre-Wirthmüller context, def. , write $\overline {\pi}$ for the natural transformation
given as the composite
where
the second is the $(f_! \dashv f^\ast)$ counit (tensored with an identity morphism).
Also write
for the $(f^\ast \dashv f_\ast)$ adjunct of the natural transformation given as the composite
where the first map exhibits $f^\ast$ as a closed monoidal functor and where the second is the $(f_! \dashv f^\ast)$-unit (under the internal hom).
see (May 05, prop. 2.11)
In a pre-Wirthmüller context, def. , the comparison maps of def. are equivalences when the argument $B \in (\mathcal{Y}, \otimes_Y, 1_Y)$ is a dualizable object.
If either of the two happens to be a natural equivalence (hence an equivalence for all arguments), then so is the other.
(May 05, prop. 2.8 and prop. 2.11)
Precisely when the pre-Wirthmüller context is a Wirthmüller context, def. , both comparison maps of def. are natural equivalences.
For all $A \in \mathcal{X}$ and $B,C \in \mathcal{Y}$ we have by the $(f_! \dashv f^\ast)$-adjunction and the tensor$\dashv$hom-adjunction a commuting diagram of the form
By naturality in $A$ and by the Yoneda lemma this shows that $\overline{\pi}$ is an equivalence precisey if $f^\ast$ is strong closed.
For $\overline{\gamma}$ the same statement follows from this with prop. .
The first natural equivalence of prop.
is often called the projection formula. In representation theory this is also sometimes called Frobenius reciprocity, though mostly that term is used for (just) the existence of the $(f_! \dashv f^\ast)$-adjunction, where in representation theory the left adjoint $f_!$ forms induced representations.
Assuming a Wirthmüller context, the projection formula has the following implications.
In the special case that both of its variables are in the image of $f^\ast$, then composing the projection formula-map from Def. with the adjunction counit in the first variable equals the adjunction unit of the full term, in that the following diagram commutes (where we are leaving the structure isomorphism of the strong monoidal functor $f^\ast$ notationally implicit):
The adjunct of the composite of the top and the right morphism is the composite of
the adjunct of the top morphism, which by Def. , is
$f^\ast A \otimes f^\ast B \overset{\eta_{f^\ast A} \otimes \eta_{f^\ast B}}{\longrightarrow} f^\ast f_! f^\ast A \otimes f^\ast f_! f^\ast B \overset{id \otimes f^\ast \epsilon_B}{\longrightarrow} f^\ast f_! f^\ast A \otimes f^\ast B$
the image under $f^\ast$ of the right morphism, which is $f^\ast f_! f^ \ast A \otimes f^\ast B \overset{f^\ast \epsilon_A \otimes id}{\longrightarrow} f^\ast A \otimes f^\ast B$.
That composite is the identity by the triangle identity in each of the two variables.
In conclusion, the left vertical morphism is the adjunct of the identity, and hence the counit as claimed.
The $(f_! \dashv f^\ast)$-adjunction counit on any object is, up to isomorphism given by the tensor product with the adjunction counit on the tensor unit:
The identification is given by the following commuting diagram:
Here
the top square is the naturality square of the adjunction counit on the right unitor,
In a pre-Wirthmüller context, def. , the functors $f_!$ and $f_\ast$ are intertwined by dualization, in that there is a natural equivalence
This is the special case of the property of $\overline{\gamma}$ in prop. for $B = 1_Y$:
With a Wirthmüller context regarded as categorical semantics for dependent linear type theory (see there), then the statement of cor. is an instance of de Morgan duality where linear dualization intertwines linear dependent sum $\sum$ and dependent product $\prod$
For more on this see also (Schreiber 14, section 3.3).
In a pre-Wirthmüller context
(Wirthmüller isomorphism)
In a Wirthmüller context, def. if $f_! 1_X$ is a dualizable object with dual $f_! C$, then there is a natural equivalence
In particular if there is $D \in \mathcal{Y}$ with
(hence if $C \simeq f^\ast D$ in the notation of prop. ) and using that by cor. , $f_\ast f^\ast \mathbb{D}B \simeq f_\ast \mathbb{D} f^\ast B \simeq \mathbb{D}(f_! f^\ast B)$ then prop. gives a natural equivalence of the form
saying that the comonad $f_! f^\ast$ commutes with dualization up to a “twist” given by tensoring with $D$.
For $\mathbf{H}$ a topos and $f \colon X \longrightarrow Y$ any morphism, then in the induced base change etale geometric morphism
the inverse image/context extension is a cartesian closed functor (see there for the proof). Therefore any base change of toposes constitutes a cartesian Wirthmüller context.
Conversely, this means that systems of Wirthmüller contexts are generalizations of categorical logic (hyperdoctrines) to non-cartesian contexts (see at dependent linear type theory).
Notice that in a cartesian Wirthmüller context duality is trivial, in that $\mathbb{D}X \simeq 1$ for all objects $X$. Therefore to the extent that the six operations yoga involves duality, it is interesting only the more non-cartesian (non-classical) the ambient Wirthmüller context is.
For instance the projection formula $\overline{\gamma}$ in def. for base change along a pointed connected type $\mathbf{B}G \to \ast$ equivalently says that genuine (Bredon) equivariant cohomology reduces to Borel equivariant cohomology when the action on the coefficients is trivial. See at equivariant cohomology for more on this.
A first step away from the Cartesian example above is the following.
Let $\mathbf{H}$ be a topos. For $X \in \mathbf{H}$ any object, write
for the category of pointed objects in the slice topos $\mathbf{H}_{/X}$. Equipped with the smash product $\wedge_X$ this is a closed symmetric monoidal category $(\mathcal{C}_X, \wedge_X, X \coprod X)$.
For $f \colon X \longrightarrow Y$ any morphism in $\mathbf{H}$, the base change inverse image $f^\ast$ restricts to a functor $f^\ast \colon \mathcal{C}_Y \longrightarrow \mathcal{C}_X$ which is a Wirthmüller context.
This appears as (Shulman 07, examples 12.13 and 13.7) and (Shulman 12, example 2.33).
For $f \colon X \longrightarrow Y$ any morphism in $\mathbf{H}$ then the base change inverse image $f^\ast \colon \mathbf{H}_{/Y} \longrightarrow \mathbf{H}_{/X}$ preserves pointedness, and the pushout functor $f_! \colon \mathbf{H}^{X/} \longrightarrow \mathbf{H}^{/Y}$ preserves co-pointedness. These two functors hence form an adjoint pair $(f_1 \dashv f^\ast) \colon \mathcal{C}_X \longrightarrow \mathcal{C}_Y$. Moreover, since colimits in the under-over category $\mathbf{H}_{/X}^{X/}$ are computed as colimits in $\mathbf{H}$ of diagrams with an initial object adjoined, and since by the Giraud axioms in the topos $\mathbf{H}$ pullback preserves these colimits, it follows that $f^\ast \colon \mathcal{C}_Y \to \mathcal{C}_X$ preserves colimits. Finally by the discussion at category of pointed objects we have that $\mathcal{C}_X$ and $\mathcal{C}_Y$ are locally presentable categories, so that by the adjoint functor theorem it follows that $f^\ast$ has also a right adjoint $f_\ast \colon \mathcal{C}_X \to \mathcal{C}_Y$.
To see that $f^\ast$ is a strong monoidal functor observe that the smash product is, by the discussion there, given by a pushout over coproducts and products in the slice topos. As above these are all preserved by pullback. Finally to see that $f^\ast$ is also a strong closed functor observe that the internal hom on pointed objects is, by the discussion there, a fiber product of cartesian internal homs. These are preserved by the above case, and the fiber product is preserved since $f^\ast$ preserves all limits. Hence $f^\ast$ preserves also the internal homs of pointed objects.
For $R$ a ring, $R Mod$ its category of modules, there is a functor
which sends a set $X$ to the closed monoidal category of $R$-modules parameterized over $X$.
This takes values in Wirthmüller morphisms. (Shulman 12, example 2.2, 2.17).
for (infinity,1)-module bundles: (Nuiten 13, Hopkins-Lurie, Schreiber 14)
A transfer context is a Wirthmüller context in which also $f_\ast$ satisfies its projection formula. In this context there is an abstract concept of Becker-Gottlieb transfer. See there for more.
In equivariant stable homotopy theory, see (May 05b).
Pull-push of quasicoherent sheaves is usually discussed as a Grothendieck context of six operations, but under some conditions it also becomes a Wirthmüller context.
Using results of Lurie this follows in the full generality of E-∞ geometry (spectral geometry).
Consider quasi-compact and quasi-separated E-∞ algebraic spaces (spectral algebraic spaces). (This includes precisely those spectral Deligne-Mumford stacks which have a scallop decomposition, see here.)
If $f \;\colon\; X \longrightarrow Y$ is a map between these which is
locally almost of finite presentation;
strongly proper;
has finite Tor-amplitude?
then the left adjoint to pullback of quasicoherent sheaves exists
(LurieProper, proposition 3.3.23)
If $f$ is
then the right adjoint exists
(LurieQC, prop. 2.5.12, LurieProper, proposition 2.5.12)
The projection formula in the dual form
for $f$ quasi-compact and quasi-separated appears as (LurieProper, remark 1.3.14).
Now if all the conditions on $f$ hold, so that $(f_! \dashv f^\ast \dashv f_\ast) \;\colon\; QCoh(X) \longrightarrow QCoh(Y)$, then passing to opposite categories $QCoh(X)^{op} \longrightarrow QCoh(Y)^{op}$ exchanges the roles of $f_!$ and $f_\ast$, makes the projection formula be as in the above discussion and hence yields a Wirthmüller context.
The existence of dualizing modules $K$
is discussed in (Lurie, Representability theorems, section 4.2.)
The Wirthmüller isomorphism in equivariant stable homotopy theory goes back to and is named after
see Theorem II.6.2 in
see also
A clear discussion of axioms of six operations and their consequences, with emphasis on the Wirthmüller isomorphisms, is in
Halvard Fausk, Po Hu, Peter May, Isomorphisms between left and right adjoints, Theory and Applications of Categories, 11 4 (2003) 107-131 [tac:11-04, pdf]
Paul Balmer, Ivo Dell'Ambrogio, Beren Sanders, Grothendieck-Neeman duality and the Wirthmüller isomorphism, Compositio Mathematica 152.8 (2016): 1740-1776 (arXiv:1501.01999)
Discussion of the original Wirthmüller isomorphism in equivariant stable homotopy theory, based on this, is in
More elaboration of the Wirthmüller context is in
Discussion in the context of pure motives includes
Wirthmüller morphisms between pointed objects and between bundles of modules are discussed in
Mike Shulman, Framed bicategories and monoidal fibrations, in Theory and Applications of Categories, Vol. 20, 2008, No. 18, pp 650-738. (arXiv:0706.1286, TAC)
Mike Shulman, Enriched indexed categories (arXiv:1212.3914)
A twisted version of the Wirthmüller isomorphism is discussed in
Discussion in E-∞ geometry is in
Last revised on May 16, 2023 at 09:15:45. See the history of this page for a list of all contributions to it.