concrete category




A concrete category is a category that looks like a category of “sets with extra structure”, that is a category of structured sets.



A concrete category is a category CC equipped with a faithful functor

U:CSet U : C \to Set

to the category Set. We say a category CC is concretizable if and only if it admits a faithful functor U:CSetU: C \to Set.


Very often it is useful to consider the case where UU is representable by some object c 0Cc_0 \in C, in that UC(c 0,)U \simeq C(c_0,-). For example, this is important for the statement of various concrete dualities induced by dual adjunctions. We say in this case that (C,U:CSet)(C, U: C \to Set) is representably concrete. By definition, the object c 0c_0 is then a separator of the category.

We remark that the existence of a left adjoint FF to U:CSetU: C \to Set implies that UU is representable by F(1)F(1). Conversely, if CC has coproducts or even just copowers, then representability of UU implies that UU has a left adjoint.


One can also consider concrete categories over any base category XX instead of necessarily over SetSet. This is the approach taken in The Joy of Cats. Then the (small) categories concrete over XX form a 2-category Cat(X)Cat(X).


The following furnish examples of concrete categories, with the first three representably concrete:

  • C=SetC = Set itself with separator c 0={}c_0 = \{\bullet\} the singleton set.

  • C=TopC = Top with the separator c 0c_0 taken to be the one-point space.

  • Any monadic functor U:CSetU: C \to Set is faithful (because it preserves equalizers and reflects isomorphisms) and has a left adjoint. As special cases, we have the usual collection of examples of concrete categories: monoids, groups, rings, algebras, etc.

A category may be concretizable in more than one way:

  • Take C C to be the category of Banach spaces with morphisms those (everywhere-defined) linear transformations with norm bounded (above) by 1 1 (so Tvv \| T v \| \leq \| v \| for all v v in the source). Then there are two versions of U U that one may use: one where U(V) U ( V ) (for V V a Banach space) consists of every vector in V V , and one where U(V) U ( V ) consists of those vectors bounded by 1 1 (so the closed unit ball in V V ). The first may seem more obvious at first, but only the second is representable (by a 1 1 -dimensional Banach space).

  • Insofar as categories such as SetSet, TopTop, Vect kVect_k, etc. admit many separators, these categories may be rendered representably concrete in a variety of ways. Indeed, the category Vect kVect_k may be monadic over SetSet in many different ways. For example, if VV is nn-dimensional, the functor hom(V,):Vect kSet\hom(V, -): Vect_k \to Set is monadic and realizes Vect kVect_k as equivalent to the category of modules over the matrix algebra hom(V,V)\hom(V, V).

  • Any Grothendieck topos is concretizable, but not necessarily (and typically not) representably concretizable. If E=Sh(C,J)E = Sh(C, J) is the category of sheaves on a small site (C,J)(C, J), we have a familiar string of faithful functors

    Sh(C,J)Set C opmonadicSet/C 0Σ C 0Set.Sh(C, J) \hookrightarrow Set^{C^{op}} \stackrel{monadic}{\to} Set/C_0 \stackrel{\Sigma_{C_0}}{\to} Set.

    But if for example EE is the category of sheaves over \mathbb{R}, then no object XX can serve as a single separator of EE, since it cannot detect differences between arrows YZY \stackrel{\to}{\to} Z whenever the support of YY is strictly contained in the support of XX.

  • A concrete category that is equipped with the structure of a site in a compatible way is a concrete site. The category of concrete sheaves on a concrete site is concrete.



Every small category CC is concretizable (since it fully and faithfully embeds in the concrete category Set C opSet^{C^{op}}).


If CC is concretizable, so is C opC^{op}.


By assumption, there is a faithful functor U op:C opSet opU^{op}: C^{op} \to Set^{op}, and hom(,2):Set opSet\hom(-, \mathbf{2}): Set^{op} \to Set is monadic.


Of course, since a category CC may possess a separator but no coseparator, it does not follow that C opC^{op} is representably concrete if CC is.



A finitely complete category is concretizable, i.e., admits a faithful functor to SetSet, if and only if it is well-powered with respect to regular subobjects.


“Only if” was proven in (Isbell). To prove it, note that if F:CDF: C\to D is a faithful functor, then it is injective on equivalence classes of regular subobjects. For suppose that m:axm\colon a \to x is the equalizer of f,g:xyf,g\colon x\rightrightarrows y, and n:bxn\colon b\to x is the equalizer of h,k:xzh,k\colon x\rightrightarrows z. If F(a)F(b)F(a) \cong F(b) as subobjects of F(x)F(x), then since fm=gmf m = g m and so F(f)F(m)=F(g)F(m)F(f)\circ F(m) = F(g)\circ F(m), we must also have F(f)F(n)=F(g)F(n)F(f)\circ F(n) = F(g)\circ F(n); hence (since FF is faithful) fn=gnf n = g n, so that nn factors through mm. Conversely, nn factors through mm, so we have aba\cong b as subobjects of xx. Since SetSet is regularly well-powered, it follows that any category admitting a faithful functor to SetSet must also be so.

(Actually, Isbell proved a more general condition which applies to categories that may lack finite limits.)

“If” was proven in (Freyd). The argument is rather more involved, passing through additive categories, and is not reproduced here.


A relatively deep application of Isbell’s result is that the classical homotopy category Ho(Top) of topological spaces is not concretizable, even though it is a quotient of TopTop which is concretizable. (Freyd 70)

A similar way to use Isbell’s result applies to show that a really vast number of model categories can not have a concrete localization at weak equivalences: see Di Liberti and Loregian, 2017


  • John Isbell, Two set-theoretical theorems in categories, Fund. Math 53 (1963)
  • Peter Freyd, Concreteness, JPAA 3 (1973)

  • Peter Freyd, Homotopy is not concrete, in The Steenrod Algebra and its Applications, Springer Lecture Notes in Mathematics Vol. 168, Springer-Verlag, 1970, Republished in: Reprints in Theory and Applications of Categories, No. 6 (2004) pp 1-10 (web)

  • Ivan di Liberti, Fosco Loregian “Homotopical algebra is not concrete.” Journal of Homotopy and Related Structures (2017): 1-15.

Last revised on August 24, 2021 at 08:52:38. See the history of this page for a list of all contributions to it.