# nLab indexed monoidal (infinity,1)-category

Indexed monoidal -categories

### Context

#### Monoidal categories

monoidal categories

# Indexed monoidal $(\infty,1)$-categories

## Idea

An indexed monoidal $(\infty,1)$-category is the (∞,1)-categorical version of an indexed monoidal category. That is, it consists of a “base” $(\infty,1)$-category $\mathcal{C}$ together with, for each $X\in \mathcal{C}$, a monoidal (∞,1)-category $Mod(X)$ varying functorially with $X$. In one of the fundamental examples, $\mathcal{C}$ is the $(\infty,1)$-category of ∞-groupoids (“spaces”), while $Mod(X)$ is that of parametrized spectra.

Indexed monoidal $(\infty,1)$-categories are conjectured to be the categorical semantics of linear homotopy type theory.

## Definition

###### Definition

An indexed monoidal $(\infty,1)$-category is

1. an (∞,1)-category $\mathcal{C}$ with finite (∞,1)-limits;

2. an (∞,1)-functor $Mod \colon \mathcal{C}^{op} \to SymMonCat_\infty$ to symmetric monoidal (∞,1)-categories;

such that

1. each $Mod(X)$ is closed (with internal hom to be denoted $[-,-]$);

2. for each $f \colon \Gamma_1 \to \Gamma_2$ in $Mor(\mathcal{C})$ the assigned (∞,1)-functor $f^\ast \colon Mod(\Gamma_2) \to Mod(\Gamma_1)$ has a left adjoint $f_!$ and a right adjoint $f_\ast$;

3. The adjunction $(f_! \dashv f^\ast)$ satisfies Frobenius reciprocity and the Beck-Chevalley condition.

In the conjectural syntax of dependent linear type theory, the objects of $\mathcal{C}$ correspond to contexts, which is why we sometimes write them as $\Gamma$. Similarly, we may sometimes write $f_!$ and $f_\ast$ as $\sum_f$ and $\prod_f$ respectively.

The statement of Frobenius reciprocity then is that

$\underset{f}{\sum} \left( X \otimes f^\ast Y \right) \simeq \left( \underset{f}{\sum} X \right) \otimes Y \,.$

## Examples

For each example we also spell out some of the abstract constructions (discussed in Structures below) realized in that model. We also include some examples of indexed monoidal 1-categories (which are a special case).

Although probably those examples should be moved to indexed monoidal category.

### Slices of a topos

###### Example

For $\mathbf{H}$ a topos, then its system $\mathbf{H}_{/(-)} \colon \mathbf{H}^{op} \to CartMonCat \to MonCat$ of slice toposes is an indexed monoidal category, hence an indexed monodial $(\infty,1)$-category.

###### Remark

This example for dependent linear type theory is extremely “non-linear”. For instance, it has almost no dualizable objects; the only one is the terminal object in each slice. Given that the formulas below for secondary integral transforms (def.) assume dualizable objects, this means that in this non-linear context there will be no nontrivial secondary integral transforms.

We now pass gradually to more and more linear examples.x

### Parameterized pointed objects

A first step away from the Cartesian example above towards more genuinely linear types is the following.

###### Definition

Let $\mathbf{H}$ be a topos. For $X \in \mathbf{H}$ any object, write

$\mathcal{C}_X \coloneqq \mathbf{H}_{/X}^{X/}$

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 $(\mathcal{C}_X, \wedge_X, X \coprod X)$.

###### Proposition

For $f \colon X \longrightarrow Y$ any morphism in $\mathbf{H}$, the base change inverse image $f^\ast$ restricts to a functor $f^\ast \colon \mathcal{C}_Y \longrightarrow \mathcal{C}_X$ and this makes

$\mathbf{H}_{/(-)}^{(-)/} \;\colon\; \mathbf{H}^{op} \longrightarrow MonCat$

an indexed monoidal category.

This appears as (Shulman 08, examples 12.13 and 13.7) and (Shulman 12, example 2.33).

###### Proof

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 $(f_! \dashv f^\ast) \colon \mathcal{C}_X \longrightarrow \mathcal{C}_Y$. 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.

### Parameterized modules

The examples of genuinely linear objects in the sense of linear algebra are the following.

###### Proposition

Let $E$ be a commutative ring. Write $E Mod$ for its category of modules. For $X\in$ Set write

$E Mod(X) \coloneqq Func(X, E Mod) \simeq (E Mod)^{\times_{\vert X \vert}}$

for the functor category from $X$ (regarded as a category) to $E Mod$, hence for the bundles of $E$-modules over the discrete space $X$. Equipped with the tensor product of modules this is a symmetric monoidal category $E Mod^{\otimes}$. This construction yields a functor

$E Mod \;\colon\; Set^{op} \longrightarrow SymMonCat \,.$

which is an indexed monoidal category.

###### Example

In the context of prop. consider $E = k$ a field. Then $k Mod \simeq Vect_k$ is the category Vect of $k$-vector spaces.

For $X \in Set$ a set, an $X$-dependent object $A \in Mod(X)\simeq Vect_k(X)$ is just a collection of ${\vert X\vert}$ vector spaces $A_x$ for $x\in X$.

For $X \in FinSet \hookrightarrow Set$ a finite set, the dependent sum and dependent product operations coincide and produce the direct sum of vector spaces:

$\underset{X}{\sum} A \simeq \underset{X}{\prod} A \simeq \underset{x\in X}{\oplus} A_x \in Vect_k(\ast) \,.$

Under this identification every morphism $f \in Mor(Set)$ with finite fibers carries a canonical untwisted fiberwise fundamental class, def. , $[f]_{canonical}$.

For

$\array{ && X_1 \times X_2 \\ & {}^{\mathllap{p_1}}\swarrow && \searrow^{\mathrlap{p_2}} \\ X_1 && && X_2 }$

a product corrrespondence of finite sets, then an integral kernel $K$ on this, according to def. , with $A_i = 1_{X_i}$, is equivalently an ${\vert X_1\vert}\times {\vert X_2\vert}$-array of elements in $k$, hence a matrix $K_{\bullet,\bullet}$.

### Parametrized module spectra

The example of parameterized modules above has an evident generalization from linear algebra to stable homotopy theory with abelian categories of modules refined to stable (∞,1)-categories of ∞-modules. Despite what this higher category theory-terminology might make the reader feel, this refinement flows naturally along the same lines as the 1-categorical situation. One may view the axiomatics of an indexed monoidal $(\infty,1)$-category as neatly characterizing precisely this intimate similarity.

###### Proposition

Let $E$ be an E-∞ ring spectrum and write $E Mod$ for its (∞,1)-category of ∞-modules. For $X \in$ ∞Grpd write

$E Mod(X) \coloneqq Func(X,E Mod)$

for the (∞,1)-category of (∞,1)-functors from $X$ (regarded as an (∞,1)-category) to $E Mod$, hence for the parameterized spectra over $X$ with $E$-∞-module structure. Equipped with the smash product of spectra this is a symmetric monoidal (∞,1)-category $E Mod^\otimes$ This construction yields an (∞,1)-functor

$E Mod \;\colon\; \infty Grpd^{op} \longrightarrow SymMonCat_\infty$

This is an indexed monoidal $(\infty,1)$-category.

###### Remark

In the case that $E = \mathbb{S}$ is the sphere spectrum, then $\mathbb{S}Mod \simeq Spectra$ is just the plain (∞,1)-category of spectra and then the above is the theory of plain parameterized spectra.

In this case the corresponding (∞,1)-Grothendieck construction is the tangent (∞,1)-topos $T(\infty Grpd)$ of ∞Grpd. This happens to be itself an (∞,1)-topos, and as such is conjecturally semantics for ordinary homotopy type theory. If ordinary homotopy type theory is enhanced with rules and axioms motivated by such models, it also becomes able to speak about stable homotopy theory and parametrized stable objects; the relation between this theory and dependent linear type theory remains to be explored.

###### Proposition

The $\Sigma$-functor, def. , in the model of parameterized $E$-module spectra, prop. , is the suspension spectrum construction $\Sigma^\infty$ followed by smash product of spectra with $E$:

$\Sigma(X) \simeq \Sigma^\infty(X)\wedge E \,.$

This is the $E$-generalized homology-spectrum of the ∞-groupoid $X$.

This construction does have a right adjoint $\Omega^\infty$, where $(\Sigma^\infty \dashv \Omega^\infty)$ is the $E$-stabilization-adjunction for ∞Grpd. Hence parameterized spectra have an exponential modality, def. .

###### Proposition

In the class of models of prop. , an indexed monoidal $(\infty,1)$-category encodes the theory of twisted generalized cohomology.

twisted generalized cohomology theory is conjecturally ∞-categorical semantics of linear homotopy type theory:

linear homotopy type theorygeneralized cohomology theoryquantum theory
linear type(module-)spectrum
multiplicative conjunctionsmash product of spectracomposite system
dependent linear typemodule spectrum bundle
Frobenius reciprocitysix operation yoga in Wirthmüller context
invertible typetwistprequantum line bundle
dependent sumgeneralized homology spectrumspace of quantum states (“bra”)
dual of dependent sumgeneralized cohomology spectrumspace of quantum states (“ket”)
linear implicationbivariant cohomologyquantum operators
exponential modalityFock space
dependent sum over finite homotopy type (of twist)suspension spectrum (Thom spectrum)
dualizable dependent sum over finite homotopy typeAtiyah duality between Thom spectrum and suspension spectrum
(twisted) self-dual typePoincaré dualityinner product
dependent sum coinciding with dependent productambidexterity, semiadditivity
dependent sum coinciding with dependent product up to invertible typeWirthmüller isomorphism
$(\sum_f \dashv f^\ast)$-counitpushforward in generalized homology
(twisted-)self-duality-induced dagger of this counit(twisted-)Umkehr map/fiber integration
linear polynomial functorcorrespondencespace of trajectories
linear polynomial functor with linear implicationintegral kernel (pure motive)prequantized Lagrangian correspondence/action functional
composite of this linear implication with daggered-counit followed by unitintegral transformmotivic/cohomological path integral
traceEuler characteristicpartition function

### Parameterized formal moduli problems

###### Definition

For $\mathbf{H}$ a differential cohesive (∞,1)-topos with infinitesimal shape modality $\Pi_{inf}$ write for any object $X\in \mathbf{H}$

$Mod(X) \hookrightarrow \mathbf{H}_{/X}^{X/}$

for the full sub-(∞,1)-category of that of pointed objects over $X$, def. , on those that are in the kernel of $\Pi_{inf}$.

###### Remark

The construction in has the interpretation as the category of generalized formal moduli problems parameterized over $X$. The genuine formal moduli problems satisfy one an extra exactness property of the kind discussed at cohesive (∞,1)-presheaf on E-∞ rings.

###### Proposition

Parameterized formal moduli problems as in def. conjecturally form semantics for non-unital linear homotopy-type theory.

### Quasicoherent sheaves of modules

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 and hence an indexed monoidal $(\infty,1)$-category

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

1. locally almost of finite presentation;

2. strongly proper;

3. has finite Tor-amplitude?

then the left adjoint to pullback of quasicoherent sheaves exists

$(f_! \dashv f^\ast) \;\colon\; QCoh(X) \stackrel{\overset{f_!}{\longrightarrow}}{\underset{f^\ast}{\longleftarrow}} QCoh(Y) \,.$

If $f$ is

• quasi-affine

$(f^\ast \dashv f_\ast) \;\colon\; QCoh(X) \stackrel{\overset{f^\ast}{\longleftarrow}}{\underset{f_\ast}{\longrightarrow}} QCoh(Y) \,.$

The projection formula in the dual form

$f_\ast A \otimes B \longrightarrow f_\ast (A\otimes f^\ast B)$

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 $K$

$D X = [X,K]$

is discussed in (Lurie, Representability theorems, section 4.2.)

### Stable homotopy theory of $G$-equivariant spectra

For a finite group, $G$, consider the spectral Mackey functors on the category of finite $G$-sets. These are genuine G-spectra (see equivariant stable homotopy theory) which together constitute a $G$-equivariant $\infty$-category (or $G$-$\infty$-category, see Parametrized Higher Category Theory and Higher Algebra). This sends any orbit $G/H$, for $H$ a subgroup of $G$, to the monoidal $\infty$-category of genuine $H$-spectra.

This forms an indexend monoidal $(\infty,1)$-category, as do other instances of $(\infty, 1)$-categories of $T$-spectra, for $T$ an orbital $\infty$-category.

## Structures in an indexed monoidal $(\infty,1)$-category

We discuss here structures (constructions) that may be defined and studied within an indexed monoidal $(\infty,1)$-category.

### Exponential modality and Fock spaces

The original axiomatics for linear type theory in (Girard 87) contain in addition to the structures corresponding to a (star-autonomous) symmetric closed monoidal category a certain (co-)modality traditionally denoted “$!$”, the exponential modality.

In (Benton 95, p.9,15, Bierman 95) it is noticed (reviewed in (Barber 97, p. 21 (26))) that a natural categorical semantics for this modality identifies it with the comonad that is induced from a strong monoidal adjunction

$(\Sigma \dashv \Omega) \;\colon\; Mod(\ast) \stackrel{\overset{\Sigma}{\leftarrow}}{\underset{\Omega}{\longrightarrow}} \mathcal{C}$

between the closed symmetric monoidal category $Mod(\ast)$ which interprets the given linear type theory and a cartesian monoidal category $\mathcal{C}$.

If there is only the strong monoidal functor $\Sigma \;\colon\; \mathcal{C} \longrightarrow Mod(\ast)$ without possibly a right adjoint $\Omega$, then (Barber 97, p. 21 (27)) speaks of the structural fragment of linear type theory.

In (Ponto-Shulman 12) it is observed that this in turn is canonically induced if $Mod(\ast)$ is the linear type theory over the trivial context $\ast$ of a dependent linear type theory (indexed closed monoidal category) with category of contexts being $\mathcal{C}$:

###### Definition

Let $Mod \colon \mathcal{C}^{op} \to MonCat$ be an indexed monoidal $(\infty,1)$-category. Then for $X \in \mathcal{C}$ an object, set

$\Sigma(X) \coloneqq \underset{X}{\sum} 1_X \in Mod(\ast)$

and for $f \;\colon\; Y \longrightarrow X$ a morphism in $\mathcal{C}$ set

$\Sigma(f) \coloneqq \Sigma(Y) = \underset{Y}{\sum} 1_Y \stackrel{\simeq}{\to} \underset{X}{\sum} \underset{f}{\sum} f^\ast 1_X \stackrel{\underset{X}{\sum}(\epsilon_f)}{\longrightarrow} \underset{X}{\sum} 1_X = \Sigma(X) \,.$

(In the typical kind of model this means to assign to a space $X$ the linear space of sections of the trivial line bundle over it.)

###### Proposition

The construction in def. gives a strong monoidal functor

$\Sigma \;\colon\; \mathcal{C} \longrightarrow Mod(\ast)$

This is (Ponto-Shulman 12, (4.3)).

###### Remark

Below in example we see that the functor $\Sigma$ in prop. is a special case of a general construction of secondary integral transforms in an indexed monoidal $(\infty,1)$-category.

This suggests the following definition.

###### Definition

Given an indexed monoidal $(\infty,1)$-category such that the functor $\Sigma$ from prop. has a strong monoidal right adjoint

$\Omega \colon Mod(\ast) \longrightarrow \mathcal{C}$

we refer to the comonad of the $(\Sigma \dashv \Omega)$-adjunction

$! \coloneqq \Sigma \circ \Omega \colon Mod(\ast) \to Mod(\ast) \,.$

as the exponential modality in the conjectural linear type theory over the point whose semantics is $Mod(\ast)$.

###### Remark

The condition in def. that $\Sigma$ (and its relative/dependent versions) has a right adjoint $\Omega$ may also be understood as saying that the dependent linear type theory satisfies the axiom of comprehension. See at comprehension – Examples – In dependent linear type theory for more.

### Dependent linear deMorgan duality

###### Definition

For $\mathcal{C}^\otimes$ a closed monoidal category with unit object $1$ and internal hom $[-,-]$ write

$\mathbb{D} \coloneqq [-,1]$

for the functor given by internal hom into the unit object. Syntactically this corresponds to the linear negation operation.

###### Proposition

(dependent linear de Morgan duality)

In an indexed monoidal $(\infty,1)$-category, linear negation, def. , intertwines dependent sum and dependent product:

$\underset{f}{\prod} \mathbb{D} \simeq \mathbb{D} \underset{f}{\sum}$

For proof see here at Wirthmüller context.

### Primary integral transforms

###### Definition

Given an indexed monoidal $(\infty,1)$-category $Mod\colon \mathcal{C}^{op}\to SymMonCat$, and given objects $X_1, X_2$ of $\mathcal{C}$ then a linear polynomial functor

$P \colon Mod(X_1) \to Mod(X_2)$

is a functor of the form

$P \simeq \underset{f_2}{\sum} \underset{g}{\prod} f_1^\ast$

for a diagram in $\mathcal{C}$ of the form

$\array{ && Y &\stackrel{g}{\longrightarrow}& Z \\ & {}^{\mathllap{f_1}}\swarrow & && & \searrow^{\mathrlap{f_2}} \\ X_1 && && && X_2 } \,.$

If here $g = id$ then this diagram is a correspondence

$\array{ && Z \\ & {}^{\mathllap{f_1}}\swarrow & & \searrow^{\mathrlap{f_2}} \\ X_1 && && X_2 }$

and the resulting $P \simeq \underset{f_2}{\sum}f_1 ^\ast$ is called a linear polynomial functor or primary integral transform.

###### Proposition

Given a correspondence as above, then the primary integral transform through it is equivalent to the pull-tensor-push operation through the product

$\underset{f_2}{\sum} \circ f_1^\ast \simeq \underset{p_2}{\sum} \circ \left( \left(f_1,f_2\right)_!\left(1_Z\right) \otimes \left(-\right)\right) \circ p_1^\ast \,.$

### Fundamental classes

###### Definition

A fiberwise twisted fundamental class $[f]$ on a morphism $f \colon X\to Y$ in $\mathcal{C}$ is

1. a choice of dualizable object $\tau \in Mod(Y)$ (the twist);

2. a choice of equivalence

$\underset{f}{\sum} f^\ast 1_X \stackrel{\simeq}{\longrightarrow} \underset{f}{\prod} f^\ast \tau \,.$

In this form this is stated in (Schreiber 14).

###### Remark

A special case of def. is obtained when the twist vanishes, $\tau = 1$, and when dependent sum and dependent product are naturally equivalent for all objects, $\sum_f \simeq \prod_f$. In this case the two adjoints to $f^\ast$ coincide to form an ambidextrous adjunction. This case is considered in (Hopkins-Lurie).

###### Remark

In view of dependent linear de Morgan duality, prop. , a fiberwise twisted fundamental class in def. is equivalently a choice of equivalence

$\underset{f}{\sum} f^\ast 1_X \stackrel{\simeq}{\longrightarrow} \mathbb{D}\underset{f}{\sum} f^\ast \mathbb{D}\tau \,,$

hence an identification of the dependent sum of the unit with the dual of the dependent sum of the twist.

###### Proposition

A twisted fiberwise fundamental class $[f]$, def. , induces for all dualizable $A\in Mod(X)$ a natural equivalence

$\mathbb{D} \underset{f}{\sum} f^\ast \mathbb{D}(A\otimes \tau) \simeq \underset{f}{\sum}f^\ast A \,.$

This is the “Wirthmüller isomorphism”.

###### Definition

For $[f]$ a twisted fiberwise fundamental class and for $A$ dualizable, write

$[f]_A \;\colon\; A \otimes \tau \stackrel{\mathbb{D}_{\epsilon_{\mathbb{D}(A \otimes \tau)}}}{\longrightarrow} \mathbb{D} \underset{f}{\sum} f^\ast (\mathbb{D}(A\otimes \tau)) \stackrel{\simeq}{\longrightarrow} \underset{f}{\sum}f^\ast A$

for the composite of the linear dual of the $(\sum_f \dashv f^\ast)$-adjunction counit and the Wirthmüller isomorphism, prop. .

###### Remark

The key point is that the morphism in def. goes in the reverse direction of the adjunction counit. In this way it plays a role in the construction of secondary integral transforms below.

### Secondary integral transforms

###### Definition

For

$\array{ && Z \\ & {}^{\mathllap{f_1}}\swarrow && \searrow^{\mathrlap{f_2}} \\ X_1 && && X_2 }$

a correspondence as in def. , then an integral kernel for it is the choice of

1. two dualizable objects $A_i \in Mod(X_i)$;

2. a morphism between their pullbacks to the correspondence space:

$f_1^\ast A_1 \stackrel{K}{\longleftarrow} f_2^\ast A_2 \,.$
###### Definition

Given

1. a correspondence $X_1 \stackrel{f_1}{\longleftarrow} Z \stackrel{f_2}{\longrightarrow} X_2$ as in def. ;

2. an integral kernel $K$ as in def. ;

3. a $\tau$-twisted fundamental class $[f_2]$ as in def.

we say that the induced secondary integral transform is the morphism

$\int_Z \Xi d \mu_f \;\colon\; \mathbb{D} \underset{X_1}{\sum} A_1 \longrightarrow \mathbb{D}\underset{X_2}{\sum} (A_2\otimes \tau)$

which is the dual (the image under $\mathbb{D}(-)$) of the following composite:

$\underset{X_1}{\sum} A_1 \stackrel{\underset{X_1}{\sum}\epsilon_{A_1}}{\longleftarrow} \underset{X_1}{\sum} \underset{f_1}{\sum} f_1^\ast A_1 \stackrel{\simeq}{\leftarrow} \underset{Z}{\sum}f_1^\ast A_1 \stackrel{\underset{Z}{\sum} K}{\longleftarrow} \underset{Z}{\sum}f_2^\ast A_2 \stackrel{\simeq}{\leftarrow} \underset{X_2}{\sum}\underset{f_2}{\sum} f_2^\ast A_2 \stackrel{\underset{X_2}{\sum}[f_2]_{A_2}}{\longleftarrow} \underset{X_2}{\sum} (A_2\otimes \tau) \,,$

where the morphism on the left is the adjunction counit, the morphism in the middle is the given integral kernel, and the morphism on the right is the given fundamental class.

In this form this appears in (Schreiber 14). Specialized to the ambidextrous case of remark , this is equivalent to the construction in (Hopkins-Lurie).

###### Remark

A correspondence $X_1 \stackrel{f_1}{\longleftarrow} Z \stackrel{f_2}{\longrightarrow} X_2$ may be thought of as a space $Z$ of “paths” or “trajectories” that connect points in $X_2$ to points in $X_1$. From this point of view definition is a linear map that takes functions on $X_2$ to functions on $X_1$ by pointwise forming a sum over paths connecting these points and adding all the contributions of the given integral kernel along these paths.

This is conceptually just how the path integral in physics is supposed to work, only that mostly it doesn’t, due to lack of a proper definition. However, at least some path integrals for topological field theories may be realized as secondary integral transforms of the above kind.

Notice that while in modal type theory the (co-)monads $(f^\ast \sum_f \dashv f^\ast \prod_f)$ are pronounced as possibility and necessity, the monad $\prod_f f^\ast$ appearing above, via def. , may be pronounced randomness, see at function monad for more.

###### Example

Consider the special case def. where the right leg of the correspondence is the identity

$\array{ && Z \\ & {}^{\mathllap{f}}\swarrow && \searrow^{\mathrlap{=}} \\ X && && Z }$

and where the integral kernel is the identity

$K = id_{1_Z} \colon id_Z^\ast 1_Z = 1_Z \to 1_Z = f^\ast 1_X \,.$

Then the associated secondary integral transform, def. , is the morphism

$\underset{X}{\sum} 1_X \stackrel{\underset{\epsilon_f}{\sum}}{\longleftarrow} \underset{X}{\sum} \underset{f}{\sum} f^\ast 1_X \stackrel{\simeq}{\longleftarrow} \underset{Z}{\sum} 1_Z \,.$

This is just the $\Sigma$-functor of def. :

$\mathbb{D}\int_Z id \, d\mu_f = \Sigma(f) \; \colon \; \Sigma(Z) \stackrel{\Sigma(f)}{\longleftarrow} \Sigma(X) \,.$
###### Proposition

Given a correspondence with integral kernel as in example , then the induced secondary integral transform according to def. is the linear function

$k^{\vert X_1\vert} \longleftarrow k^{\vert X_2\vert}$

represented by that matrix.

## References

Plain linear type theory originates in

The categorical interpretation of Girard’s $!$-modality as a comonad is due to

• P. N. Benton, G. M. Bierman, Martin Hyland, Valeria de Paiva, Term assignment for intuitionistic linear logic, Technical Report 262, Computer Laboratory, University of Cambridge, August 1992.

and that this is naturally to be thought of as arising from a monoidal adjunction between the closed symmetric monoidal category and a cartesian closed category is due to

• G. Bierman, On Intuitionistic Linear Logic PhD thesis, Computing Laboratory, University of Cambridge, 1995 (pdf)

• N. Benton, A mixed linear and non-linear logic; proofs, terms and models, In Proceedings of Computer Science Logic ‘94, vol. 933 of Lecture Notes in Computer Science. Verlag, June 1995. (pdf)

A review of all this and further discussion is in

• Andrew Graham Barber, Linear Type Theories, Semantics and Action Calculi, 1997 (web, pdf)

The 1-categorical version of indexed closed monoidal categories is discussed in

Comments on the formalization of secondary integral transforms and path integral quantization in an indexed monoidal $(\infty,1)$-category are in

Last revised on March 4, 2021 at 05:56:54. See the history of this page for a list of all contributions to it.