decidable object


A decidable object is the categorical rendering of the notion of a decidable set from computability theory. This corresponds to the algebraic and topological concepts separable , respectively unramified object, as pointed out by Lawvere.


An object XX of a coherent category 𝒞\mathcal{C} is called decidable if its equality relation Δ X:XX×X,\Delta_X:X\to X\times X\quad , is complemented, as a subobject of X×XX\times X . A morphism f:ABf:A\to B is called decidable if it is a decidable object in the slice category 𝒞/B\mathcal{C}/B.


  • For an object XX this means that in the internal logic of the category, it is true that “for any x,yXx,y\in X , either x=yx=y or xyx\neq y”.

  • 00 and 11 are always decidable, and so is every natural numbers object NN in a topos. A subobject of a decidable object is decidable.

  • Decidable maps f:ABf:A\to B in the opposite of the category of commutative rings CommRing opCommRing^{op} are precisely the separable BB-algebras AA.

  • Of course, in a Boolean category, every object is decidable. Conversely in a topos \mathcal{E}, or more generally a coherent category with a subobject classifier, every object is decidable precisely if \mathcal{E} is Boolean.

  • In constructive mathematics, where Set is not assumed Boolean, one says that a set XX has decidable equality if it is a decidable object of Set\Set.

  • A decidable subobject simply means a complemented subobject. Again, in constructive mathematics, a decidable subobject in Set\Set is called a decidable subset.

  • An object XX in a topos \mathcal{E} is called anti-decidable if ¬¬(x=y)\neg\neg (x=y) in the internal language of \mathcal{E} holds for all x,yXx,y\in X. A formula φ\varphi is called almost decidable iff ¬φ¬¬φ\neg\varphi\vee\neg\neg\varphi holds and an object XX is called almost decidable if x=yx=y is almost decidable for xXx\in X.


  • B. P. Chisala, M.-M. Mawanda, Counting Measure for Kuratowski Finite Parts and Decidability , Cah.Top.Géom.Diff.Cat. XXXII 4 (1991) pp.345-353. (pdf)

  • A. Carboni, G. Janelidze, Decidable (=separable) objects and morphisms in lextensive categories , JPAA 110 (1996) pp.219-240.

  • Peter Johnstone, Sketches of an Elephant vols. I,II, Oxford UP 2002.

Last revised on September 5, 2014 at 15:11:03. See the history of this page for a list of all contributions to it.