Not signed in

Want to take part in these discussions? Sign in if you have an account, or apply for one below

• Username
• Password
• Remember me

• Sign in using OpenID
• Remember me

• Forgot your password?
• Apply for membership

2-category 2-category-theory abelian-categories adjoint algebra algebraic algebraic-geometry algebraic-topology analysis analytic-geometry arithmetic arithmetic-geometry bundles calculus categorical categories category category-theory chern-weil-theory cohesion cohesive-homotopy-type-theory cohomology colimits combinatorics complex-geometry computable-mathematics computer-science constructive cosmology deformation-theory descent diagrams differential differential-cohomology differential-equations differential-geometry digraphs duality elliptic-cohomology enriched fibration finite foundations functional-analysis functor galois-theory gauge-theory gebra geometric-quantization geometry graph graphs gravity grothendieck group group-theory harmonic-analysis higher higher-algebra higher-category-theory higher-differential-geometry higher-geometry higher-lie-theory higher-topos-theory homological homological-algebra homotopy homotopy-theory homotopy-type-theory index-theory integration integration-theory internal-categories k-theory lie lie-theory limit limits linear linear-algebra locale localization logic manifolds mathematics measure-theory modal modal-logic model model-category-theory monads monoidal monoidal-category-theory morphism motives motivic-cohomology nforum nlab noncommutative noncommutative-geometry number number-theory of operads operator operator-algebra order-theory pages pasting philosophy physics pro-object probability probability-theory quantization quantum quantum-field quantum-field-theory quantum-mechanics quantum-physics quantum-theory question representation representation-theory riemannian-geometry scheme schemes set set-theory sheaf simplicial space spin-geometry stable-homotopy-theory string string-theory superalgebra supergeometry svg symplectic-geometry synthetic-differential-geometry terminology theory topology topos topos-theory type type-theory universal variational-calculus

1. • CommentRowNumber1.
   • CommentAuthormaxsnew
   • CommentTimeOct 24th 2018
   • PermaLink
   Author: maxsnew
   Format: MarkdownItexAdd page structure, idea section. I preserved most of the original page under a definition section. <a href="https://ncatlab.org/nlab/revision/diff/actegory/6">diff</a>, <a href="https://ncatlab.org/nlab/revision/actegory/6">v6</a>, <a href="https://ncatlab.org/nlab/show/actegory">current</a>

   Add page structure, idea section.

   I preserved most of the original page under a definition section.

   diff, v6, current

2. • CommentRowNumber2.
   • CommentAuthormaxsnew
   • CommentTimeOct 24th 2018
   • PermaLink
   Author: maxsnew
   Format: MarkdownItexI'm looking at these "actegories" because I noticed that models of [[call-by-push-value]] can be described as actions of a (cartesian) monoidal category on a category with a dual adjunction between the two categories compatible with the action. As an aside, I don't like the name actegory and would just prefer action of a monoidal category. Searching for applications for CS/programming I'm curious if there are any common uses of actions of a monoidal category? The only idea I have is that as monoidal categories are the "right" place to define monoids, an action of a monoidal category is the "right" place to define the action of a monoid: If C acts on D you can define a monoid in C acting on an object of D. Does anyone have any examples where things are fruitfully framed this way?

   I’m looking at these “actegories” because I noticed that models of call-by-push-value can be described as actions of a (cartesian) monoidal category on a category with a dual adjunction between the two categories compatible with the action. As an aside, I don’t like the name actegory and would just prefer action of a monoidal category.

   Searching for applications for CS/programming I’m curious if there are any common uses of actions of a monoidal category?

   The only idea I have is that as monoidal categories are the “right” place to define monoids, an action of a monoidal category is the “right” place to define the action of a monoid: If C acts on D you can define a monoid in C acting on an object of D. Does anyone have any examples where things are fruitfully framed this way?

3. • CommentRowNumber3.
   • CommentAuthorUrs
   • CommentTimeOct 24th 2018
   • (edited Oct 24th 2018)
   • PermaLink
   Author: Urs
   Format: MarkdownItexhyperlinked "actions" (!) <a href="https://ncatlab.org/nlab/revision/diff/actegory/7">diff</a>, <a href="https://ncatlab.org/nlab/revision/actegory/7">v7</a>, <a href="https://ncatlab.org/nlab/show/actegory">current</a>

   hyperlinked “actions” (!)

   diff, v7, current

4. • CommentRowNumber4.
   • CommentAuthorMike Shulman
   • CommentTimeOct 24th 2018
   • PermaLink
   Author: Mike Shulman
   Format: MarkdownItexI don't like the word "actegory" either. One example of this "right place to define monoids" thing is the definition of the [[bar construction]]. If $V$ is a monoidal category acting on $C$, and $D\in V$ is a monoid and $M\in C$ is a $D$-module, then there is a simplicial object $B_\bullet(D,D,M) \in C^{\Delta^{op}}$ where $B_n(D,D,M) = D^{\otimes (n+1)} \odot M$, whose realization is a "resolution" of $M$ *qua* $D$-module. At this level the gain is perhaps not obvious, since this bar construction factors through the "universal" action of $End(C)$ on $C$, with $D$ corresponding to the monad (= monoid in $End(C)$) $T_D(X) \coloneqq D\odot X$. But when you talk about two-sided bar constructions $B(N,D,M)$ and "coend-like" bar constructions $B(D,H)$ I think the extra generality can be useful. Although it's been a while since I thought about this sort of thing much.

   I don’t like the word “actegory” either.

   One example of this “right place to define monoids” thing is the definition of the bar construction. If $V$ is a monoidal category acting on $C$, and $D\in V$ is a monoid and $M\in C$ is a $D$-module, then there is a simplicial object $B_\bullet(D,D,M) \in C^{\Delta^{op}}$ where $B_n(D,D,M) = D^{\otimes (n+1)} \odot M$, whose realization is a “resolution” of $M$ qua $D$-module. At this level the gain is perhaps not obvious, since this bar construction factors through the “universal” action of $End(C)$ on $C$, with $D$ corresponding to the monad (= monoid in $End(C)$) $T_D(X) \coloneqq D\odot X$. But when you talk about two-sided bar constructions $B(N,D,M)$ and “coend-like” bar constructions $B(D,H)$ I think the extra generality can be useful. Although it’s been a while since I thought about this sort of thing much.

5. • CommentRowNumber5.
   • CommentAuthorSam Staton
   • CommentTimeOct 24th 2018
   • PermaLink
   Author: Sam Staton
   Format: MarkdownItexHi -- The Kelly-Janelidze paper has a nice idea: an enriched category with copowers is the same thing as an actegory that has a right adjoint in the appropriate argument. This leads to a simpler or even more "natural" presentation for enriched categories with copowers. If you are looking for examples, Rasmus Mogelberg and I used it extensively in <a href="https://arxiv.org/pdf/1403.1477.pdf">linear usage of state</a> and I found it useful in <a href="http://www.cs.ox.ac.uk/people/samuel.staton/papers/freyd-lawvere-2014.pdf">Freyd categories are enriched Lawvere theories</a>. There is also a really nice characterization of enrichment with _finite_ copowers in a locally D-presentable smc, in terms of actions (Prop 2.7 in my Freyd cats paper, but see also "Enrichment through variation"). Plenty of other people have also found this kind of thing useful including Marcelo Fiore, Paul Levy and Paul-Andre Mellies. This might all be related to call-by-push-value.

   Hi – The Kelly-Janelidze paper has a nice idea: an enriched category with copowers is the same thing as an actegory that has a right adjoint in the appropriate argument. This leads to a simpler or even more “natural” presentation for enriched categories with copowers. If you are looking for examples, Rasmus Mogelberg and I used it extensively in linear usage of state and I found it useful in Freyd categories are enriched Lawvere theories.

   There is also a really nice characterization of enrichment with finite copowers in a locally D-presentable smc, in terms of actions (Prop 2.7 in my Freyd cats paper, but see also “Enrichment through variation”).

   Plenty of other people have also found this kind of thing useful including Marcelo Fiore, Paul Levy and Paul-Andre Mellies. This might all be related to call-by-push-value.

6. • CommentRowNumber6.
   • CommentAuthorRichard Williamson
   • CommentTimeOct 24th 2018
   • PermaLink
   Author: Richard Williamson
   Format: MarkdownItexActions of monoidal categories are very important in geometric representation theory. For example, everything under the name 2-representation theory in the sense of Rouquier is about this: $sl_2$-categorification, 2-Kac-Moody algebras, etc.

   Actions of monoidal categories are very important in geometric representation theory. For example, everything under the name 2-representation theory in the sense of Rouquier is about this: $sl_2$-categorification, 2-Kac-Moody algebras, etc.

7. • CommentRowNumber7.
   • CommentAuthormaxsnew
   • CommentTimeOct 25th 2018
   • PermaLink
   Author: maxsnew
   Format: MarkdownItexThanks everyone for the examples. Unsurprisingly Sam's are the most understandable to me at the moment. I came to my current model of CBPV from the Enriched Effect Calculus work which I described somewhat here: https://ncatlab.org/nlab/show/call-by-push-value#as_an_adjoint_logic . I sent you an email with more details!

   Thanks everyone for the examples. Unsurprisingly Sam’s are the most understandable to me at the moment. I came to my current model of CBPV from the Enriched Effect Calculus work which I described somewhat here: https://ncatlab.org/nlab/show/call-by-push-value#as_an_adjoint_logic . I sent you an email with more details!

8. • CommentRowNumber8.
   • CommentAuthorSam Staton
   • CommentTimeAug 15th 2019
   • PermaLink
   Author: Sam Staton
   Format: MarkdownItexmention connection to enrichment <a href="https://ncatlab.org/nlab/revision/diff/actegory/8">diff</a>, <a href="https://ncatlab.org/nlab/revision/actegory/8">v8</a>, <a href="https://ncatlab.org/nlab/show/actegory">current</a>

   mention connection to enrichment

   diff, v8, current

9. • CommentRowNumber9.
   • CommentAuthormattecapu
   • CommentTimeJul 19th 2021
   • PermaLink
   Author: mattecapu
   Format: MarkdownItexAdded a partial definition <a href="https://ncatlab.org/nlab/revision/diff/actegory/11">diff</a>, <a href="https://ncatlab.org/nlab/revision/actegory/11">v11</a>, <a href="https://ncatlab.org/nlab/show/actegory">current</a>

   Added a partial definition

   diff, v11, current