Coalgebra


In mathematics, coalgebras or cogebras are structures that are dual to unital associative algebras. The axioms of unital associative algebras can be formulated in terms of commutative diagrams. Turning all arrows around, one obtains the axioms of coalgebras.
Every coalgebra, by duality, gives rise to an algebra, but not in general the other way. In [|finite dimensions], this duality goes in both directions.
Coalgebras occur naturally in a number of contexts.
There are also F-coalgebras, with important applications in computer science.

Formal definition

Formally, a coalgebra over a field K is a vector space C over K together with K-linear maps Δ: C → C ⊗ C and ε: C → K such that
  2. .
Equivalently, the following two diagrams commute:
In the first diagram, C ⊗ is identified with ⊗ C; the two are naturally isomorphic. Similarly, in the second diagram the naturally isomorphic spaces C, C ⊗ K and K ⊗ C are identified.
The first diagram is the dual of the one expressing associativity of algebra multiplication ; the second diagram is the dual of the one expressing the existence of a multiplicative identity. Accordingly, the map Δ is called the comultiplication of C and ε is the of C.

Examples

Take an arbitrary set S and form the K-vector space C = K with basis S, as follows. The elements of this vector space C are those functions from S to K that map all but finitely many elements of S to zero; identify the element s of S with the function that maps s to 1 and all other elements of S to 0. Define
By linearity, both Δ and ε can then uniquely be extended to all of C. The vector space C becomes a coalgebra with comultiplication Δ and counit ε.
As a second example, consider the polynomial ring K in one indeterminate X. This becomes a coalgebra if for all n ≥ 0 one defines:
Again, because of linearity, this suffices to define Δ and ε uniquely on all of K. Now K is both a unital associative algebra and a coalgebra, and the two structures are compatible. Objects like this are called bialgebras, and in fact most of the important coalgebras considered in practice are bialgebras. Examples include Hopf algebras and Lie bialgebras.
The tensor algebra and the exterior algebra are further examples of coalgebras.
The singular homology of a topological space forms a graded coalgebra whenever the Künneth isomorphism holds, e.g. if the coefficients are taken to be a field.
If C is the K-vector space with basis, consider Δ: C → C ⊗ C is given by
and ε: C → K is given by
In this situation, is a coalgebra known as trigonometric coalgebra.
For a locally finite poset P with set of intervals J, define the incidence coalgebra C with J as basis and comultiplication for x < z
The intervals of length zero correspond to points of P and are group-like elements.

Finite dimensions

In finite dimensions, the duality between algebras and coalgebras is closer: the dual of a finite-dimensional algebra is a coalgebra, while the dual of a finite-dimensional coalgebra is a algebra. In general, the dual of an algebra may not be a coalgebra.
The key point is that in finite dimensions, and are isomorphic.
To distinguish these: in general, algebra and coalgebra are dual notions, while for finite dimensions, they are dual objects.
If A is a finite-dimensional unital associative K-algebra, then its K-dual A∗ consisting of all K-linear maps from A to K is a coalgebra. The multiplication of A can be viewed as a linear map, which when dualized yields a linear map. In the finite-dimensional case, is naturally isomorphic to, so this defines a comultiplication on A∗. The counit of A∗ is given by evaluating linear functionals at 1.

Sweedler notation

When working with coalgebras, a certain notation for the comultiplication simplifies the formulas considerably and has become quite popular. Given an element c of the coalgebra, there exist elements c and c in C such that
In Sweedler's notation,, this is abbreviated to
The fact that ε is a counit can then be expressed with the following formula
The coassociativity of Δ can be expressed as
In Sweedler's notation, both of these expressions are written as
Some authors omit the summation symbols as well; in this sumless Sweedler notation, one writes
and
Whenever a variable with lowered and parenthesized index is encountered in an expression of this kind, a summation symbol for that variable is implied.

Further concepts and facts

A coalgebra is called co-commutative if, where is the K-linear map defined by for all c, d in C. In Sweedler's sumless notation, C is co-commutative if and only if
for all c in C.
A group-like element is an element x such that and. Contrary to what this naming convention suggests the group-like elements do not always form a group and in general they only form a set. The group-like elements of a Hopf algebra do form a group. A primitive element is an element x that satisfies. The primitive elements of a Hopf algebra form a Lie algebra.
If and are two coalgebras over the same field K, then a coalgebra morphism from C1 to C2 is a K-linear map such that and.
In Sweedler's sumless notation, the first of these properties may be written as:
The composition of two coalgebra morphisms is again a coalgebra morphism, and the coalgebras over K together with this notion of morphism form a category.
A linear subspace I in C is called a coideal if and. In that case, the quotient space C/I becomes a coalgebra in a natural fashion.
A subspace D of C is called a subcoalgebra if ; in that case, D is itself a coalgebra, with the restriction of ε to D as counit.
The kernel of every coalgebra morphism is a coideal in C1, and the image is a subcoalgebra of C2. The common isomorphism theorems are valid for coalgebras, so for instance C1/ker is isomorphic to im.
If A is a finite-dimensional unital associative K-algebra, then A∗ is a finite-dimensional coalgebra, and indeed every finite-dimensional coalgebra arises in this fashion from some finite-dimensional algebra. Under this correspondence, the commutative finite-dimensional algebras correspond to the cocommutative finite-dimensional coalgebras. So in the finite-dimensional case, the theories of algebras and of coalgebras are dual; studying one is equivalent to studying the other. However, relations diverge in the infinite-dimensional case: while the K-dual of every coalgebra is an algebra, the K-dual of an infinite-dimensional algebra need not be a coalgebra.
Every coalgebra is the sum of its finite-dimensional subcoalgebras, something that is not true for algebras. Abstractly, coalgebras are generalizations, or duals, of finite-dimensional unital associative algebras.
Corresponding to the concept of representation for algebras is a corepresentation or comodule.