In category theory

An equalizer is a limit

eqexgfy\operatorname{eq}\underset{\quad e \quad}{\to}x\underoverset{\quad g \quad}{f}{\rightrightarrows}y

over a parallel pair i.e. of the diagram of the shape

{xgfy}.\left\lbrace x \underoverset{\quad g \quad}{f}{\rightrightarrows} y \right\rbrace.

(See also fork diagram).

This means that for f:xyf : x \to y and g:xyg : x \to y two parallel morphisms in a category CC, their equalizer is, if it exists

The dual concept is that of coequalizer.

In type theory

In type theory the equalizer

PAgfBP \to A \stackrel{\overset{f}{\longrightarrow}}{\underset{g}{\longrightarrow}} B

is given by the dependent sum over the dependent equality type

P a:A(f(a)=g(a)).P \simeq \sum_{a : A} (f(a) = g(a)).




A category has equalizers if it has binary products and pullbacks.


For SfgTS \stackrel{\overset{g}{\longrightarrow}}{\underset{f}{\longrightarrow}} T the given diagram, form the pullback along the diagonal morphism of TT:

eq(f,g) S (f,g) T (id,id) T×T.\array{ eq(f,g) &\longrightarrow& S \\ \big\downarrow && \big\downarrow {}^{\mathrlap{(f, g)}} \\ T &\underset{(id, id)}{\longrightarrow}& T \times T }.

One checks that the horizontal morphism eq(f,g)Seq(f,g) \to S equalizes ff and gg and that it does so universally.


If a category has equalizers and (finite) products, then it has (finite) limits.

For the finite case, we may say equivalently:


If a category has equalizers, binary products and a terminal object, then it has finite limits.


(Eckmann and Hilton EH, Proposition 1.3.) Let e:EXe: E \rightarrow X be an arrow in a category 𝒞\mathcal{C} which is an equaliser of a pair of arrows of 𝒞\mathcal{C}. Then ee is a monomorphism.


If g,h:AEg,h : A \rightarrow E are arrows of 𝒞\mathcal{C} such that eg=ehe \circ g = e \circ h, then it follows immediately from the uniqueness part of the universal property of an equaliser that g=hg = h.


Equalizers were defined in the paper

for any finite collection of parallel morphisms. The paper refers to them as left equalizers, whereas coequalizers are referred to as right equalizers.

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