Inmathematics, more specifically incategory theory, auniversal property is a property that characterizesup to anisomorphism the result of some constructions. Thus, universal properties can be used for defining some objects independently from the method chosen for constructing them. For example, the definitions of theintegers from thenatural numbers, of therational numbers from the integers, of thereal numbers from the rational numbers, and ofpolynomial rings from thefield of their coefficients can all be done in terms of universal properties. In particular, the concept of universal property allows a simple proof that allconstructions of real numbers are equivalent: it suffices to prove that they satisfy the same universal property.

Technically, a universal property is defined in terms ofcategories andfunctors by means of auniversal morphism (see§ Formal definition, below). Universal morphisms can also be thought more abstractly asinitial or terminal objects of acomma category (see§ Connection with comma categories, below).

Universal properties occur almost everywhere in mathematics, and the use of the concept allows the use of general properties of universal properties for easily proving some properties that would need boring verifications otherwise. For example, given acommutative ringR, thefield of fractions of thequotient ring ofR by aprime idealp can be identified with theresidue field of thelocalization ofR atp; that is${\displaystyle R_p/p{R}_p\cong \mathrm{Frac}(R/p)}$ (all these constructions can be defined by universal properties).

Other objects that can be defined by universal properties include: allfree objects,direct products anddirect sums,free groups,free lattices,Grothendieck group,completion of a metric space,completion of a ring,Dedekind–MacNeille completion,product topologies,Stone–Čech compactification,tensor products,inverse limit anddirect limit,kernels andcokernels,quotient groups,quotient vector spaces, and otherquotient spaces.

Before giving a formal definition of universal properties, we offer some motivation for studying such constructions.

• The concrete details of a given construction may be messy, but if the construction satisfies a universal property, one can forget all those details: all there is to know about the construction is already contained in the universal property. Proofs often become short and elegant if the universal property is used rather than the concrete details. For example, thetensor algebra of avector space is slightly complicated to construct, but much easier to deal with by its universal property.
• Universal properties define objects uniquely up to a uniqueisomorphism.^[1] Therefore, one strategy to prove that two objects are isomorphic is to show that they satisfy the same universal property.
• Universal constructions are functorial in nature: if one can carry out the construction for every object in a categoryC then one obtains afunctor onC. Furthermore, this functor is aright or left adjoint to the functorU used in the definition of the universal property.^[2]
• Universal properties occur everywhere in mathematics. By understanding their abstract properties, one obtains information about all these constructions and can avoid repeating the same analysis for each individual instance.

To understand the definition of a universal construction, it is important to look at examples. Universal constructions were not defined out of thin air, but were rather defined after mathematicians began noticing a pattern in many mathematical constructions (see Examples below). Hence, the definition may not make sense to one at first, but will become clear when one reconciles it with concrete examples.

Let${\displaystyle F:\mathcal{C}\to \mathcal{D}}$ be a functor between categories${\displaystyle \mathcal{C}}$ and${\displaystyle \mathcal{D}}$. In what follows, let${\displaystyle X}$ be an object of${\displaystyle \mathcal{D}}$,${\displaystyle A}$ and${\displaystyle A^{\prime }}$ be objects of${\displaystyle \mathcal{C}}$, and${\displaystyle h:A\to A^{\prime }}$ be a morphism in${\displaystyle \mathcal{C}}$.

Then, the functor${\displaystyle F}$ maps${\displaystyle A}$,${\displaystyle A^{\prime }}$ and${\displaystyle h}$ in${\displaystyle \mathcal{C}}$ to${\displaystyle F(A)}$,${\displaystyle F(A^{\prime })}$ and${\displaystyle F(h)}$ in${\displaystyle \mathcal{D}}$.

Auniversal morphism from${\displaystyle X}$ to${\displaystyle F}$ is a unique pair${\displaystyle (A,u:X\to F(A))}$ in${\displaystyle \mathcal{D}}$ which has the following property, commonly referred to as auniversal property:

For any morphism of the form${\displaystyle f:X\to F(A^{\prime })}$ in${\displaystyle \mathcal{D}}$, there exists aunique morphism${\displaystyle h:A\to A^{\prime }}$ in${\displaystyle \mathcal{C}}$ such that the following diagramcommutes:

We candualize this categorical concept. Auniversal morphism from${\displaystyle F}$ to${\displaystyle X}$ is a unique pair${\displaystyle (A,u:F(A)\to X)}$ that satisfies the following universal property:

For any morphism of the form${\displaystyle f:F(A^{\prime })\to X}$ in${\displaystyle \mathcal{D}}$, there exists aunique morphism${\displaystyle h:A^{\prime }\to A}$ in${\displaystyle \mathcal{C}}$ such that the following diagram commutes:

Note that in each definition, the arrows are reversed. Both definitions are necessary to describe universal constructions which appear in mathematics; but they also arise due to the inherent duality present in category theory.In either case, we say that the pair${\displaystyle (A,u)}$ which behaves as above satisfies a universal property.

Universal morphisms can be described more concisely as initial and terminal objects in acomma category (i.e. one where morphisms are seen as objects in their own right).

Let${\displaystyle F:\mathcal{C}\to \mathcal{D}}$ be a functor and${\displaystyle X}$ an object of${\displaystyle \mathcal{D}}$. Then recall that the comma category${\displaystyle (X\downarrow F)}$ is the category where

• Objects are pairs of the form${\displaystyle (B,f:X\to F(B))}$, where${\displaystyle B}$ is an object in${\displaystyle \mathcal{C}}$
• A morphism from${\displaystyle (B,f:X\to F(B))}$ to${\displaystyle (B^{\prime },f^{\prime }:X\to F(B^{\prime }))}$ is given by a morphism${\displaystyle h:B\to B^{\prime }}$ in${\displaystyle \mathcal{C}}$ such that the diagram commutes:

Now suppose that the object${\displaystyle (A,u:X\to F(A))}$ in${\displaystyle (X\downarrow F)}$ is initial. Thenfor every object${\displaystyle (A^{\prime },f:X\to F(A^{\prime }))}$, there exists a unique morphism${\displaystyle h:A\to A^{\prime }}$ such that the following diagram commutes.

Note that the equality here simply means the diagrams are the same. Also note that the diagram on the right side of the equality is the exact same as the one offered in defining auniversal morphism from${\displaystyle X}$ to${\displaystyle F}$. Therefore, we see that a universal morphism from${\displaystyle X}$ to${\displaystyle F}$ is equivalent to an initial object in the comma category${\displaystyle (X\downarrow F)}$.

Conversely, recall that the comma category${\displaystyle (F\downarrow X)}$ is the category where

• Objects are pairs of the form${\displaystyle (B,f:F(B)\to X)}$ where${\displaystyle B}$ is an object in${\displaystyle \mathcal{C}}$
• A morphism from${\displaystyle (B,f:F(B)\to X)}$ to${\displaystyle (B^{\prime },f^{\prime }:F(B^{\prime })\to X)}$ is given by a morphism${\displaystyle h:B\to B^{\prime }}$ in${\displaystyle \mathcal{C}}$ such that the diagram commutes:

Suppose${\displaystyle (A,u:F(A)\to X)}$ is a terminal object in${\displaystyle (F\downarrow X)}$. Then for every object${\displaystyle (A^{\prime },f:F(A^{\prime })\to X)}$, there exists a unique morphism${\displaystyle h:A^{\prime }\to A}$ such that the following diagrams commute.

The diagram on the right side of the equality is the same diagram pictured when defining auniversal morphism from${\displaystyle F}$ to${\displaystyle X}$. Hence, a universal morphism from${\displaystyle F}$ to${\displaystyle X}$ corresponds with a terminal object in the comma category${\displaystyle (F\downarrow X)}$.

Below are a few examples, to highlight the general idea. The reader can construct numerous other examples by consulting the articles mentioned in the introduction.

Let${\displaystyle \mathcal{C}}$ be thecategory of vector spaces${\displaystyle K}$-Vect over afield${\displaystyle K}$ and let${\displaystyle \mathcal{D}}$ be the category ofalgebras${\displaystyle K}$-Alg over${\displaystyle K}$ (assumed to beunital andassociative). Let

${\displaystyle U}$ :${\displaystyle K}$-Alg →${\displaystyle K}$-Vect

be theforgetful functor which assigns to each algebra its underlying vector space.

Given anyvector space${\displaystyle V}$ over${\displaystyle K}$ we can construct thetensor algebra${\displaystyle T(V)}$. The tensor algebra is characterized by the fact:

“Any linear map from${\displaystyle V}$ to an algebra${\displaystyle A}$ can be uniquely extended to analgebra homomorphism from${\displaystyle T(V)}$ to${\displaystyle A}$.”

This statement is an initial property of the tensor algebra since it expresses the fact that the pair${\displaystyle (T(V),i)}$, where${\displaystyle i:V\to U(T(V))}$ is the inclusion map, is a universal morphism from the vector space${\displaystyle V}$ to the functor${\displaystyle U}$.

Since this construction works for any vector space${\displaystyle V}$, we conclude that${\displaystyle T}$ is a functor from${\displaystyle K}$-Vect to${\displaystyle K}$-Alg. This means that${\displaystyle T}$ isleft adjoint to the forgetful functor${\displaystyle U}$ (see the section below onrelation to adjoint functors).

Acategorical product can be characterized by a universal construction. For concreteness, one may consider theCartesian product inSet, thedirect product inGrp, or theproduct topology inTop, where products exist.

Let${\displaystyle X}$ and${\displaystyle Y}$ be objects of a category${\displaystyle \mathcal{C}}$ with finite products. The product of${\displaystyle X}$ and${\displaystyle Y}$ is an object${\displaystyle X}$ ×${\displaystyle Y}$ together with two morphisms

${\displaystyle {\pi }_1}$ :${\displaystyle X\times Y\to X}$

${\displaystyle {\pi }_2}$ :${\displaystyle X\times Y\to Y}$

such that for any other object${\displaystyle Z}$ of${\displaystyle \mathcal{C}}$ and morphisms${\displaystyle f:Z\to X}$ and${\displaystyle g:Z\to Y}$ there exists a unique morphism${\displaystyle h:Z\to X\times Y}$ such that${\displaystyle f={\pi }_1\circ h}$ and${\displaystyle g={\pi }_2\circ h}$.

To understand this characterization as a universal property, take the category${\displaystyle \mathcal{D}}$ to be theproduct category${\displaystyle \mathcal{C}\times \mathcal{C}}$ and define thediagonal functor

${\displaystyle \mathrm{\Delta }:\mathcal{C}\to \mathcal{C}\times \mathcal{C}}$

by${\displaystyle \mathrm{\Delta }(X)=(X,X)}$ and${\displaystyle \mathrm{\Delta }(f:X\to Y)=(f,f)}$. Then${\displaystyle (X\times Y,({\pi }_1,{\pi }_2))}$ is a universal morphism from${\displaystyle \mathrm{\Delta }}$ to the object${\displaystyle (X,Y)}$ of${\displaystyle \mathcal{C}\times \mathcal{C}}$: if${\displaystyle (f,g)}$ is any morphism from${\displaystyle (Z,Z)}$ to${\displaystyle (X,Y)}$, then it must equala morphism${\displaystyle \mathrm{\Delta }(h:Z\to X\times Y)=(h,h)}$ from${\displaystyle \mathrm{\Delta }(Z)=(Z,Z)}$to${\displaystyle \mathrm{\Delta }(X\times Y)=(X\times Y,X\times Y)}$ followed by${\displaystyle ({\pi }_1,{\pi }_2)}$. As a commutative diagram:

For the example of the Cartesian product inSet, the morphism${\displaystyle ({\pi }_1,{\pi }_2)}$ comprises the two projections${\displaystyle {\pi }_1(x,y)=x}$ and${\displaystyle {\pi }_2(x,y)=y}$. Given any set${\displaystyle Z}$ and functions${\displaystyle f,g}$ the unique map such that the required diagram commutes is given by${\displaystyle h=⟨x,y⟩(z)=(f(z),g(z))}$.^[3]

Categorical products are a particular kind oflimit in category theory. One can generalize the above example to arbitrary limits and colimits.

Let${\displaystyle \mathcal{J}}$ and${\displaystyle \mathcal{C}}$ be categories with${\displaystyle \mathcal{J}}$ asmallindex category and let${\displaystyle {\mathcal{C}}^{\mathcal{J}}}$ be the correspondingfunctor category. Thediagonal functor

${\displaystyle \mathrm{\Delta }:\mathcal{C}\to {\mathcal{C}}^{\mathcal{J}}}$

is the functor that maps each object${\displaystyle N}$ in${\displaystyle \mathcal{C}}$ to the constant functor${\displaystyle \mathrm{\Delta }(N):\mathcal{J}\to \mathcal{C}}$ (i.e.${\displaystyle \mathrm{\Delta }(N)(X)=N}$ for each${\displaystyle X}$ in${\displaystyle \mathcal{J}}$ and${\displaystyle \mathrm{\Delta }(N)(f)=1_N}$ for each${\displaystyle f:X\to Y}$ in${\displaystyle \mathcal{J}}$) and each morphism${\displaystyle f:N\to M}$ in${\displaystyle \mathcal{C}}$ to the natural transformation${\displaystyle \mathrm{\Delta }(f):\mathrm{\Delta }(N)\to \mathrm{\Delta }(M)}$ in${\displaystyle {\mathcal{C}}^{\mathcal{J}}}$ defined as, for every object${\displaystyle X}$ of${\displaystyle \mathcal{J}}$, the component$${\displaystyle \mathrm{\Delta }(f)(X):\mathrm{\Delta }(N)(X)\to \mathrm{\Delta }(M)(X)=f:N\to M}$$at${\displaystyle X}$. In other words, the natural transformation is the one defined by having constant component${\displaystyle f:N\to M}$ for every object of${\displaystyle \mathcal{J}}$.

Given a functor${\displaystyle F:\mathcal{J}\to \mathcal{C}}$ (thought of as an object in${\displaystyle {\mathcal{C}}^{\mathcal{J}}}$), thelimit of${\displaystyle F}$, if it exists, is nothing but a universal morphism from${\displaystyle \mathrm{\Delta }}$ to${\displaystyle F}$. Dually, thecolimit of${\displaystyle F}$ is a universal morphism from${\displaystyle F}$ to${\displaystyle \mathrm{\Delta }}$.

Defining a quantity does not guarantee its existence. Given a functor${\displaystyle F:\mathcal{C}\to \mathcal{D}}$ and an object${\displaystyle X}$ of${\displaystyle \mathcal{D}}$, there may or may not exist a universal morphism from${\displaystyle X}$ to${\displaystyle F}$. If, however, a universal morphism${\displaystyle (A,u)}$ does exist, then it is essentially unique. Specifically, it is uniqueup to auniqueisomorphism: if${\displaystyle (A^{\prime },u^{\prime })}$ is another pair, then there exists a unique isomorphism${\displaystyle k:A\to A^{\prime }}$ such that${\displaystyle u^{\prime }=F(k)\circ u}$.This is easily seen by substituting${\displaystyle (A,u^{\prime })}$ in the definition of a universal morphism.

It is the pair${\displaystyle (A,u)}$ which is essentially unique in this fashion. The object${\displaystyle A}$ itself is only unique up to isomorphism. Indeed, if${\displaystyle (A,u)}$ is a universal morphism and${\displaystyle k:A\to A^{\prime }}$ is any isomorphism then the pair${\displaystyle (A^{\prime },u^{\prime })}$, where${\displaystyle u^{\prime }=F(k)\circ u}$ is also a universal morphism.

The definition of a universal morphism can be rephrased in a variety of ways. Let${\displaystyle F:\mathcal{C}\to \mathcal{D}}$ be a functor and let${\displaystyle X}$ be an object of${\displaystyle \mathcal{D}}$. Then the following statements are equivalent:

• ${\displaystyle (A,u)}$ is a universal morphism from${\displaystyle X}$ to${\displaystyle F}$
• ${\displaystyle (A,u)}$ is aninitial object of thecomma category${\displaystyle (X\downarrow F)}$
• ${\displaystyle (A,F(\bullet )\circ u)}$ is arepresentation of${\displaystyle {\text{Hom}}_{\mathcal{D}}(X,F(-))}$, where its components${\displaystyle (F(\bullet )\circ u{)}_B:{\text{Hom}}_{\mathcal{C}}(A,B)\to {\text{Hom}}_{\mathcal{D}}(X,F(B))}$ are defined by

$${\displaystyle (F(\bullet )\circ u{)}_B(f:A\to B):X\to F(B)=F(f)\circ u:X\to F(B)}$$

for each object${\displaystyle B}$ in${\displaystyle \mathcal{C}.}$

The dual statements are also equivalent:

• ${\displaystyle (A,u)}$ is a universal morphism from${\displaystyle F}$ to${\displaystyle X}$
• ${\displaystyle (A,u)}$ is aterminal object of the comma category${\displaystyle (F\downarrow X)}$
• ${\displaystyle (A,u\circ F(\bullet ))}$ is a representation of${\displaystyle {\text{Hom}}_{\mathcal{D}}(F(-),X)}$, where its components${\displaystyle (u\circ F(\bullet ){)}_B:{\text{Hom}}_{\mathcal{C}}(B,A)\to {\text{Hom}}_{\mathcal{D}}(F(B),X)}$ are defined by

$${\displaystyle (u\circ F(\bullet ){)}_B(f:B\to A):F(B)\to X=u\circ F(f):F(B)\to X}$$

for each object${\displaystyle B}$ in${\displaystyle \mathcal{C}.}$

Suppose${\displaystyle (A_1,u_1)}$ is a universal morphism from${\displaystyle X_1}$ to${\displaystyle F}$ and${\displaystyle (A_2,u_2)}$ is a universal morphism from${\displaystyle X_2}$ to${\displaystyle F}$. By the universal property of universal morphisms, given any morphism${\displaystyle h:X_1\to X_2}$ there exists a unique morphism${\displaystyle g:A_1\to A_2}$ such that the following diagram commutes:

Ifevery object${\displaystyle X_i}$ of${\displaystyle \mathcal{D}}$ admits a universal morphism to${\displaystyle F}$, then the assignment${\displaystyle X_i\mapsto A_i}$ and${\displaystyle h\mapsto g}$ defines a functor${\displaystyle G:\mathcal{D}\to \mathcal{C}}$. The maps${\displaystyle u_i}$ then define anatural transformation from${\displaystyle 1_{\mathcal{D}}}$ (the identity functor on${\displaystyle \mathcal{D}}$) to${\displaystyle F\circ G}$. The functors${\displaystyle (F,G)}$ are then a pair ofadjoint functors, with${\displaystyle G}$ left-adjoint to${\displaystyle F}$ and${\displaystyle F}$ right-adjoint to${\displaystyle G}$.

Similar statements apply to the dual situation of terminal morphisms from${\displaystyle F}$. If such morphisms exist for every${\displaystyle X}$ in${\displaystyle \mathcal{C}}$ one obtains a functor${\displaystyle G:\mathcal{C}\to \mathcal{D}}$ which is right-adjoint to${\displaystyle F}$ (so${\displaystyle F}$ is left-adjoint to${\displaystyle G}$).

Indeed, all pairs of adjoint functors arise from universal constructions in this manner. Let${\displaystyle F}$ and${\displaystyle G}$ be a pair of adjoint functors with unit${\displaystyle \eta }$ and co-unit${\displaystyle ϵ}$ (see the article onadjoint functors for the definitions). Then we have a universal morphism for each object in${\displaystyle \mathcal{C}}$ and${\displaystyle \mathcal{D}}$:

• For each object${\displaystyle X}$ in${\displaystyle \mathcal{C}}$,${\displaystyle (F(X),{\eta }_X)}$ is a universal morphism from${\displaystyle X}$ to${\displaystyle G}$. That is, for all${\displaystyle f:X\to G(Y)}$ there exists a unique${\displaystyle g:F(X)\to Y}$ for which the following diagrams commute.
• For each object${\displaystyle Y}$ in${\displaystyle \mathcal{D}}$,${\displaystyle (G(Y),{ϵ}_Y)}$ is a universal morphism from${\displaystyle F}$ to${\displaystyle Y}$. That is, for all${\displaystyle g:F(X)\to Y}$ there exists a unique${\displaystyle f:X\to G(Y)}$ for which the following diagrams commute.

Universal constructions are more general than adjoint functor pairs: a universal construction is like an optimization problem; it gives rise to an adjoint pair if and only if this problem has a solution for every object of${\displaystyle \mathcal{C}}$ (equivalently, every object of${\displaystyle \mathcal{D}}$).

Universal properties of various topological constructions were presented byPierre Samuel in 1948. They were later used extensively byBourbaki. The closely related concept of adjoint functors was introduced independently byDaniel Kan in 1958.

• Free object
• Natural transformation
• Adjoint functor
• Monad (category theory)
• Variety of algebras
• Cartesian closed category

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