# Cofree coalgebra

In algebra, the cofree coalgebra of a vector space or module is a coalgebra analog of the free algebra of a vector space. The cofree coalgebra of any vector space over a field exists, though it is more complicated than one might expect by analogy with the free algebra.

## Definition

If V  is a vector space over a field F, then the cofree coalgebra C (V), of V, is a coalgebra together with a linear map C (V)→V, such that any linear map from a coalgebra X to V factors through a coalgebra homomorphism from X to C (V). In other words, the functor C  is right adjoint to the forgetful functor from coalgebras to vector spaces.

The cofree coalgebra of a vector space always exists, and is unique up to canonical isomorphism.

Cofree cocommutative coalgebras are defined in a similar way, and can be constructed as the largest cocommutative coalgebra in the cofree coalgebra.

## Construction

C (V) may be constructed as a completion of the tensor coalgebra T(V) of V. For kN = {0, 1, 2, ...}, let TkV denote the k-fold tensor power of V:

${\displaystyle T^{k}V=V^{\otimes k}=V\otimes V\otimes \cdots \otimes V,}$

with T0V = F, and T1V = V. Then T(V) is the direct sum of all TkV:

${\displaystyle T(V)=\bigoplus _{k\in {\mathbb {N} }}T^{k}V={\mathbb {F} }\oplus V\oplus (V\otimes V)\oplus (V\otimes V\otimes V)\oplus \cdots .}$

In addition to the graded algebra structure given by the tensor product isomorphisms TjVTkVTj+kV for j, kN, T(V) has a graded coalgebra structure Δ : T(V) → T(V) ⊗ T(V) defined by extending

${\displaystyle \Delta (v_{1}\otimes \dots \otimes v_{k}):=\sum _{j=0}^{k}(v_{1}\otimes \dots \otimes v_{j})\otimes (v_{j+1}\otimes \dots \otimes v_{k})}$

by linearity to all of T(V). This coproduct does not make T(V) into a bialgebra, but is instead dual to the algebra structure on T(V), where V denotes the dual vector space of linear maps VF. Here an element of T(V) defines a linear form on T(V) using the nondegenerate pairings

${\displaystyle T^{k}V\times T^{k}V^{*}\to \mathbb {F} }$

induced by evaluation, and the duality between the coproduct on T(V) and the product on T(V) means that

${\displaystyle \Delta (f)(a\otimes b)=f(ab).}$

This duality extends to a nondegenerate pairing

${\displaystyle {\hat {T}}(V)\times T(V^{*})\to \mathbb {F} ,}$

where

${\displaystyle {\hat {T}}(V)=\prod _{k\in \mathbb {N} }T^{k}V}$

is the direct product of the tensor powers of V. (The direct sum T(V) is the subspace of the direct product for which only finitely many components are nonzero.) However, the coproduct Δ on T(V) only extends to a linear map

${\displaystyle {\hat {\Delta }}\colon {\hat {T}}(V)\to {\hat {T}}(V){\hat {\otimes }}{\hat {T}}(V)}$

with values in the completed tensor product, which in this case is

${\displaystyle {\hat {T}}(V){\hat {\otimes }}{\hat {T}}(V)=\prod _{j,k\in \mathbb {N} }T^{j}V\otimes T^{k}V,}$

and contains the tensor product as a proper subspace:

${\displaystyle {\hat {T}}(V)\otimes {\hat {T}}(V)=\{X\in {\hat {T}}(V){\hat {\otimes }}{\hat {T}}(V):\exists \,k\in {\mathbb {N} },f_{j},g_{j}\in {\hat {T}}(V){\text{ s.t. }}X={\textstyle \sum }_{j=0}^{k}(f_{j}\otimes g_{j})\}.}$

The completed tensor coalgebra C (V) is the largest subspace C  satisfying

${\displaystyle T(V)\subseteq C\subseteq {\hat {T}}(V){\text{ and }}{\hat {\Delta }}(C)\subseteq C\otimes C\subseteq {\hat {T}}(V){\hat {\otimes }}{\hat {T}}(V),}$

which exists because if C1 and C2 satisfiy these conditions, then so does their sum C1 + C2.

It turns out[1] that C (V) is the subspace of all representative elements:

${\displaystyle C(V)=\{f\in {\hat {T}}(V):{\hat {\Delta }}(f)\in {\hat {T}}(V)\otimes {\hat {T}}(V)\}.}$

Furthermore, by the finiteness principle for coalgebras, any fC (V) must belong to a finite dimensional subcoalgebra of C (V). Using the duality pairing with T(V), it follows that fC (V) if and only if the kernel of f on T(V) contains a two-sided ideal of finite codimension. Equivalently,

${\displaystyle C(V)=\bigcup \{I^{0}\subseteq {\hat {T}}(V):I\triangleleft T(V^{*}),\,\mathrm {codim} \,I<\infty \}}$

is the union of annihilators I 0 of finite codimension ideals I  in T(V), which are isomorphic to the duals of the finite dimensional algebra quotients T(V)/I.

### Example

When V = F, T(V) is the polynomial algebra F[t] in one variable t, and the direct product

${\displaystyle {\hat {T}}(V)=\prod _{k\in \mathbb {N} }T^{k}V}$

may be identified with the vector space F[[τ]] of formal power series

${\displaystyle \sum _{j\in \mathbb {N} }a_{j}\tau ^{j}}$

in an indeterminate τ. The coproduct Δ on the subspace F[τ] is determined by

${\displaystyle \Delta (\tau ^{k})=\sum _{i+j=k}\tau ^{i}\otimes \tau ^{j}}$

and C (V) is the largest subspace of F[[τ]] on which this extends to a coalgebra structure.

The duality F[[τ]] × F[t] → F is determined by τj(tk) = δjk so that

${\displaystyle {\biggl (}\sum _{j\in \mathbb {N} }a_{j}\tau ^{j}{\biggr )}{\biggl (}\sum _{k=0}^{N}b_{k}t^{k}{\biggr )}=\sum _{k=0}^{N}a_{k}b_{k}.}$

Putting t=τ-1, this is the constant term in the product of two formal Laurent series. Thus, given a polynomial p(t) with leading term tN, the formal Laurent series

${\displaystyle {\frac {\tau ^{j-N}}{p(\tau ^{-1})}}={\frac {\tau ^{j}}{\tau ^{N}p(\tau ^{-1})}}}$

is a formal power series for any jN, and annihilates the ideal I(p) generated by p for j < N. Since F[t]/I(p) has dimension N, these formal power series span the annihilator of I(p). Furthermore, they all belong to the localization of F[τ] at the ideal generated by τ, i.e., they have the form f(τ)/g(τ) where f and g are polynomials, and g has nonzero constant term. This is the space of rational functions in τ which are regular at zero. Conversely, any proper rational function annihilates an ideal of the form I(p).

Any nonzero ideal of F[t] is principal, with finite dimensional quotient. Thus C (V) is the sum of the annihilators of the principal ideals I(p), i.e., the space of rational functions regular at zero.

## References

1. Hazewinkel 2003
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