nLab dagger category


\dagger categories


The definition of a category effectively enforces an ordering on the “0-faces” – the source and target objects – of every 1-cell (every morphism). In many cases this is essential, in that there is no way to regard the generic morphism afba \stackrel{f}{\to} b in the category as a morphism from bb to aa instead.

But there are many categories for which this is not the case, where every morphism naturally only comes with the information of an unordered pair {a,b}\{a,b \} of objects, without any prejudice on which is to be regarded as source and which as target. An important general example is:

  • the category Spans(C)Spans(C) of spans in a category CC with pullbacks, or dually, the category CoSpans(C)CoSpans(C) of cospans in a category CC with pushouts.

More concrete examples are:

  • categories of cobordisms (but notice that cobordisms are naturally regarded as cospans which makes this a special case of the above example);

  • the category Hilb of Hilbert spaces, where for every linear map f:H 1H 2f : H_1 \to H_2 we also have the adjoint map (in the sense of Hilbert spaces, not in the categorical sense) f :H 2H 1f^\dagger : H_2 \to H_1 (but notice that according to groupoidification this is also essentially to be regarded as a special case of categories of spans).

A dagger structure on a category is extra structure which encodes the idea of removing the ordering information on the 0-faces of 1-cells in a category: it is a contravariant functor which sends every morphism f:abf : a \to b to a morphism going the other way, f :baf^\dagger : b \to a.

The notation and terminology here is motivated from the example Hilb of Hilbert spaces, where f f^\dagger is traditionally the notion for the adjoint of a linear map ff. The canonical †-structure on Hilb and on nCob is crucial in quantum field theory where it is used to encode the idea of unitarity:

a unitary functorial QFT of dimension nn is supposed to be a functor nCobHilbn Cob \to Hilb which respects the †-structure on both sides.


In Wikipedia a dagger category is said to be the same as involutive category or category with involution. However, Springer’s Encyclopedia of Mathematics requires that a “category with involution” is also compatibly enriched over posets.

In enriched category theory, involutive categories have also been called symmetric categories. Sometimes the involution is required to be strict in the sense that the dagger is an equality rather than an isomorphism: in line with horizontal categorification, one could argue these should be called commutative categories, since a one-object category with such an involution is a commutative monoid.


Dagger categories

With a family of functions


A dagger category or \dagger-category CC is a category with a function () :Hom C(A,B)Hom C(B,A)(-)^\dagger: Hom_C(A,B) \to Hom_C(B,A) for every object A,BOb(C)A,B \in Ob(C), such that

  • for every AOb(C)A \in Ob(C), (1 A) =1 A(1_A)^\dagger = 1_A,
  • for every A,BOb(C)A,B \in Ob(C) and every fHom C(A,B)f \in Hom_C(A,B) and gHom C(B,C)g \in Hom_C(B,C), (gf) =f g (g \circ f)^\dagger = f^\dagger \circ g^\dagger,
  • and for every A,BOb(C)A,B \in Ob(C) and every fHom C(A,B)f \in Hom_C(A,B), ((f) ) =f((f)^\dagger)^\dagger = f.

With a function


Given a category CC with a type or class of objects Ob(C)Ob(C) and a set of morphisms Mor(C)Mor(C) with source and target functions s:Mor(C)Ob(C)s:Mor(C) \to Ob(C) and t:Mor(C)Ob(C)t:Mor(C) \to Ob(C), CC is a dagger category if it has a function () :Mor(C)Mor(C)(-)^\dagger:Mor(C) \to Mor(C) such that

  • for every term f:Mor(C)f:Mor(C), s(f)=t(f )s(f) = t(f^\dagger)
  • for every term f:Mor(C)f:Mor(C), t(f)=s(f )t(f) = s(f^\dagger)
  • for every term f:Mor(C)f:Mor(C), (f ) =f(f^\dagger)^\dagger = f
  • for every term a:Ob(C)a:Ob(C), id(a) =id(a)id(a)^\dagger = id(a)
  • for every term f:Mor(C)f:Mor(C) and g:Mor(C)g:Mor(C) such that t(f)= Ds(g)t(f) =_D s(g), (gf) =f g (g \circ f)^\dagger = f^\dagger \circ g^\dagger.

With a functor


A dagger category is a category CC equipped with a contravariant endofunctor, hence an ordinary functor from the opposite category C opC^{op} of CC to CC itself

:C opC \dagger : C^{op} \to C


  1. is the identity-on-objects,

  2. is an involution op=id C\dagger^{op} \circ \dagger = \mathrm{id}_C.

This definition, which is perhaps the most concise, does rely on a notion of identity-on-objects functor. This is no problem in most foundations for mathematics, although it violates the principle of equivalence (except for strict categories). In homotopy type theory (or more generally intensional type theory) it doesn’t make sense to talk about “being the identity on objects” as a property of a given functor, but one can still define an “identity-on-objects endofunctor” (covariant or contravariant) as a basic notion; see identity-on-objects functor for details. When unwound for \dagger-categories, this yields the above “family of functions” definition.

Special morphisms


A morphism ff in a †-category is called a unitary morphism if its †-adjoint equals its inverse:

f =f 1. f^\dagger = f^{-1} \,.

For the purpose of considering what makes two objects of a \dagger-category equivalent, one should not consider all isomorphisms (invertible morphisms) but rather all unitary isomorphisms.

The unitary isomorphisms form a groupoid, which may be regarded as the dagger-core of the \dagger-category.

For example, in Hilb, there are many invertible linear operators, but only those of norm 11 (the invertible isometries) are unitary.


A morphism ff in a †-category is called a self-adjoint morphism if it equals its †-adjoint

f =f. f^\dagger = f \,.

The category of †-categories


Given two \dagger-categories AA and BB, a \dagger -functor F:ABF : A \to B consists of a function F 0:Ob(A)Ob(B)F_0 : Ob(A) \to Ob(B) with a function F a,b:Hom A(a,b)Hom B(Fa,Fb)F_{a,b}:Hom_A(a,b) \to Hom_B(F a,F b) for every object a,b:Ob(A)a,b:Ob(A), where F a,bF_{a,b} is generally also denoted as FF, such that

  • for every object aOb(A)a \in Ob(A), F(1 a)=1 FaF(1_a)=1_{F a},
  • for every object a,b,cOb(A)a,b,c \in Ob(A) and morphisms fHom A(a,b)f \in Hom_A(a,b) and gHom A(b,c)g \in Hom_A(b,c), F(g Af)=Fg BFfF(g \circ_A f) = F g \circ_B F f
  • for every object a,bOb(A)a,b \in Ob(A) and morphism fHom A(a,b)f \in Hom_A(a,b), F(f A)=(Ff) BF(f^{\dagger_A}) = (F f)^{\dagger_B}.

More concisely, one can say that a \dagger-functor (A,)(B,)(A,\dagger) \to (B,\ddagger) is a functor F:ABF : A \to B of the underlying categories that commutes with the \dagger-structures in that F=F opF \circ \dagger = \ddagger \circ F^{op}. This appears to violate the principle of equivalence, since it is a strict equality of functors; however, once \dagger and \ddagger are known to be the identity on objects (whatever that means), the two functors FF\circ \dagger and F op\ddagger \circ F^{op} automatically agree on objects.

A natural transformation between \dagger-functors is just a natural transformation of the underlying functors.


The †-adjoint η *\eta^* of a natural transformation

η:FG \eta : F \to G

between two †-functors F,G:(C,)(D,)F, G : (C,\dagger) \to (D,\ddagger) is given by the componentwise \ddagger-adjoint:

(η *) a:=(η a) . (\eta^*)_a := (\eta_a)^\ddagger \,.

To check that η *\eta^* is indeed a natural transformation η *:GF\eta^* : G \to F consider f:abf : a \to b any morphism in CC and f :baf^\dagger : b \to a its \dagger-adjoint and let

F(a) η a G(a) F(f ) G(f ) F(b) η b G(b) \array{ F(a) &\stackrel{\eta_a}{\to}& G(a) \\ \uparrow^{\mathrlap{F(f^\dagger)}} && \uparrow^{\mathrlap{G(f^\dagger)}} \\ F(b) &\stackrel{\eta_b}{\to}& G(b) }

be the corresponding naturality square of η\eta. Taking the \ddagger-adjoint of the entire diagram yields

F(a) η a G(a) F(f ) G(f ) F(b) η b G(b)=F(a) η a G(a) F(f) G(f) F(b) η b G(b) \array{ F(a) &\stackrel{\eta_a^\ddagger}{\leftarrow}& G(a) \\ \downarrow^{\mathrlap{F(f^\dagger)^{\ddagger}}} && \downarrow^{\mathrlap{G(f^\dagger)^{\ddagger}}} \\ F(b) &\stackrel{\eta_b^{\ddagger}}{\leftarrow}& G(b) } \;\;\; = \;\;\; \array{ F(a) &\stackrel{\eta_a^\ddagger}{\leftarrow}& G(a) \\ \downarrow^{\mathrlap{F(f)}} && \downarrow^{\mathrlap{G(f)}} \\ F(b) &\stackrel{\eta_b^{\ddagger}}{\leftarrow}& G(b) }

by the fact that FF and GG are †-functors. This is the naturality square over ff of η *:GF\eta^* : G \to F.


Write DagCatDagCat for the category whose objects are †-categories and whose morphisms are †-functors.

For (C,)(C,\dagger) and (D,)(D,\dagger) two †-categories, write ([(C,),(D,)] dag,)DagCat([(C,\dagger),(D,\ddagger)]_{dag}, \star) \in DagCat for the †-category whose objects are †-functors, whose morphisms are natural transformations, with the †-operation :ηη *\star : \eta \mapsto \eta^* as above.


The assignment ((C,),(D,))[(C,),(D,)] dag,)((C,\dagger),(D,\ddagger)) \mapsto [(C,\dagger),(D,\ddagger)]_{dag}, \star) extends to an internal hom-functor

[,]:DagCat op×DagCatDagCat [-,-] : DagCat^{op} \times DagCat \to DagCat

that makes DagCatDagCat into a cartesian closed category.


This follows step-by-step the standard proof that Cat is cartesian closed, while observing that each step respects the respect for †-structures.

To indicate the main point, let C,DC, D and EE be †-categories and consider a functor F:C×DEF : C \times D \to E. For (f:c 1c 2)C(f : c_1 \to c_2) \in C and (g:d 1d 2)D(g : d_1 \to d_2) \in D we have natural assignments

(c 1,d 1) (Id,g) (c 1,d 2) (f,Id) (f,g) (f,Id) (c 2,d 1) (Id,g) (c 2,d 2)F(c 1,d 1) F(Id,g) F(c 1,d 2) F(f,Id) F(f,Id) F(c 2,d 1) F(Id,g) f(c 2,d 2) \array{ (c_1, d_1) &\stackrel{(Id,g)}{\to}& (c_1, d_2) \\ \downarrow^{\mathrlap{(f,Id)}} &\searrow^{(f,g)}& \downarrow^{\mathrlap{(f,Id)}} \\ (c_2, d_1) &\stackrel{(Id,g)}{\to}& (c_2, d_2) } \;\;\;\;\; \mapsto \;\;\;\;\; \array{ F(c_1, d_1) &\stackrel{F(Id,g)}{\to}& F(c_1, d_2) \\ \downarrow^{\mathrlap{F(f,Id)}} && \downarrow^{\mathrlap{F(f,Id)}} \\ F(c_2, d_1) &\stackrel{F(Id,g)}{\to}& f(c_2, d_2) }

that respect daggering all morphisms, in the evident way.

Keeping d 1d_1 and d 2d_2 fixed, respectively this makes F(,d 1),F(,d 2):CEF(-,d_1), F(-,d_2) : C \to E †-functors. We see from the diagrams that F(,(d 1g)d 2)F(-,(d_1 \stackrel{g}{\to}) d_2) is a natural transformation between these †-functors, and the fact that FF intertwines the dagger operation of DD with that of EE means FF regarded as a functor D[C,E]D \to [C,E] intertwines the †-structures of DD and [D,E] dag[D,E]_{dag}, by the above definition.


  • The category Rel of sets and relations is a †-category, taking dagger as relational converse.

  • More generally, let CC be a category with pullbacks and let Span 1(C)Span_1(C) be the 1-category of spans up to isomorphism: its morphisms are spans with one leg labeled as source, the other labeled as target. Then the functor :Span 1(C) opSpan 1(C)\dagger : Span_1(C)^{op} \to Span_1(C) which just exchanges this labeling is a †-structure on Span 1(C)Span_1(C).

  • (G)\mathcal{R}(G), the category of unitary representations of a (discrete) group GG and intertwining maps, is a †-category. For an intertwiner ϕ:RS\phi : R \rightarrow S, let ϕ :SR\phi^\dagger : S \rightarrow R be the adjoint of ϕ\phi in Hilb.

  • The category of couplings between probability spaces.

  • Every symmetric proset is a thin †-category.


Monoidal †-categories


A \dagger -2-poset is a \dagger-category that is also a 2-poset. Examples of \dagger-2-posets include allegories and bicategories of relations.

Model structure on †-categories

the following is based on a remark by Andre Joyal, posted to the CategoryTheory mailing list on Jan 5, 2010, with a follow-up on Jan 6.

Consider †-categories from the point of view of homotopy theory.

Recall that the category Cat of small categories naturally admits the model category structure called the folk model structure on Cat.

The category of small †-categories DCatDCat also admits a “natural” model category structure:

  • †-functor f:ABf:A \to B is a weak equivalence iff it is

    • full and faithful;

    • and unitary surjective, meaning that every object of BB is unitary isomorphic to an object in the image of the functor ff;

  • the cofibrations and the trivial fibrations are as in Cat;

  • fibrations are the unitary isofibration: maps having the right lifting property for unitary isomorphisms.

The forgetful functor DCatCatDCat \to Cat is a right adjoint but it is not a right Quillen functor with respect to the natural model structures on these categories.

Moreover, a forgetful functor XStrucCatXStruc \to Cat should reflect weak equivalences in addition to preserving them. The forgetful functor DCatCatDCat\to Cat preserves weak equivalences but it does not reflect them. Because two objects in a †-category can be isomorphic without been unitary isomorphic.

In other words the forgetful functor DCatCatDCat\to Cat is wrong. This may explains why a †-category cannot be regarded as a category equipped a homotopy invariant structure, as discussed in more detail in the example sections of the entry principle of equivalence.

But the notion of †-category is perfectly reasonable from an homotopy theoretic point of view. This is because the model category DCatDCat is a combinatorial model category. It follows, by a general result, that the notion of †-category is homotopy essentially algebraic. There is a homotopy limit sketch whose category of models (in spaces) is Quillen equivalent to the model category DCatDCat. This is true also for the model category Cat.

\dagger-simplicial set

the following is based on a remark by Andre Joyal, posted to the CategoryTheory mailing list on Jan 6, 2010

A †-simplicial set can be defined to be a simplicial set XX equipped with an involutive isomorphism :XX op\dagger :X\to X^{op} which is the identity on 0-cells. The category of †-simplicial sets (and dagger preserving maps) is the category of presheaves on the category whose objects are the ordinals [n][n], but where the maps [m][n][m]\to [n] are order reversing or preserving.



the following is based on a remark by Andre Joyal, posted to the CategoryTheory mailing list on Jan 6, 2010

There should be a notion of †-quasi-category based on \dagger-simplicial sets as ordinary quasi-categories are based on ordinary simplicial sets.



Relation to anti-involutive monoids

The horizontal categorification of an anti-involutive monoid is a \dagger-category

Relation to symmetric prosets

Every symmetric proset is a thin \dagger-category, and the groupoidal categorification of a symmetric proset is a \dagger-category, with the \dagger operation being the groupoidal categorification of the symmetric property.

Relation to star-algebras

The category convolution algebra of a dagger category is naturally a star-algebra. The star-involution is given by pullback of functions along the \dagger-functor.


Quantum mechanics in terms of \dagger-compact categories

The finite-dimensional Hilbert spaces of interest in quantum information theory and quantum computation form \dagger-categories that are also compact closed categories (dagger compact categories), a fact that allow a string diagram-calculus for quantum mechanics in terms of †-compact categories which motivates much of the contemporary discussion of dagger-categories, starting with Abramsky & Coecke 2004.

algebraic structureoidification
pointed magma with an endofunctionsetoid/Bishop set
unital magmaunital magmoid
anti-involutive monoiddagger category
associative quasigroupassociative quasigroupoid
flexible magmaflexible magmoid
alternative magmaalternative magmoid
absorption monoidabsorption category
cancellative monoidcancellative category
rigCMon-enriched category
nonunital ringAb-enriched semicategory
nonassociative ringAb-enriched unital magmoid
nonassociative algebralinear magmoid
nonassociative unital algebraunital linear magmoid
nonunital algebralinear semicategory
associative unital algebralinear category
C-star algebraC-star category
differential algebradifferential algebroid
flexible algebraflexible linear magmoid
alternative algebraalternative linear magmoid
Lie algebraLie algebroid
monoidal poset2-poset
strict monoidal groupoid?strict (2,1)-category
strict 2-groupstrict 2-groupoid
strict monoidal categorystrict 2-category
monoidal groupoid(2,1)-category
monoidal category2-category/bicategory



Motivated by the example of finite-dimensional Hilbert spaces, the concept of \dagger-categories appears in the form of strongly compact closed categories in

  • Samson Abramsky, Bob Coecke, around Prop. 7.3 A categorical semantics of quantum protocols, Proceedings of the 19th IEEE conference on Logic in Computer Science (LiCS’04), IEEE Computer Science Press (2004) [arXiv:quant-ph/0402130]

then named dagger compact closed categories in

and as such became the foundation of quantum information theory via dagger-compact categories.

Exposition in this context of quantum physics:

  • John Baez, The \star-category of Hilbert spaces, Section 3 in: Quantum quandaries: a category-theoretic perspective, in Structural Foundations of Quantum Gravity, Oxford U. Press (2006) 240-265 [web, arXiv:quant-ph/0404040]

and of quantum information theory via dagger-compact categories:

But the notion of plain dagger categories has been considered much earlier, reinvented many times under different names (see Karvonen 2018 §1.2 for more historical references) such as under the name “categories with involution”:

  • Saunders Mac Lane, pp. 1045 of: An Algebra of Additive Relations, Proceedings of the National Academy of Sciences of the United States of America 47 7 (1961) 1043-1051 [jstor:71127]

  • Dieter Puppe, §1 in Korrespondenzen in abelschen Kategorien, Mathematische Annalen 148 (1962) 1–30 [doi:10.1007/BF01438388]

  • Mark S. Burgin, Categories with involution, and correspondences in γ\gamma-categories, Trudy Moskovskogo Matematicheskogo Obshchestva 22 (1970) 161–228 [mathnet:mmo235]

Vaguely related is the notion of “symmetric category” is (analogous to symmetric proset or symmetric bicategory)

Still, dedicated discussion of these notions only appears later under the name of dagger-categories:

concerning limits:

concerning monads (such as the Frobenius quantum reader monad):

concerning abstract characterization:

Certain specially nice \dagger-categories, such as C \ * C^\* -categories and modular tensor categories, play an important role in topological quantum field theory and the theory of quantum groups, see the references there for more.

Higher dagger categories

On dagger higher categories:

Three definitions of dagger ( , 1 ) (\infty,1) -categories have been proposed by:

Generalization to dagger ( , n ) (\infty,n) -categories:

Last revised on March 17, 2024 at 22:35:35. See the history of this page for a list of all contributions to it.