# dual of a coalgebra is an algebra, the

Let $R$ be a commutative ring with unity. Suppose we have a coassociative coalgebra $(C,\Delta)$ and an associative algebra $A$, both over $R$. Since $C$ and $A$ are both $R$-modules, it follows that $\mathrm{Hom}_{R}({C},{A})$ is also an $R$-module. But in fact we can give it the structure of an associative $R$-algebra  . To do this, we use the convolution product  . Namely, given morphisms $f$ and $g$ in $\mathrm{Hom}_{R}({C},{A})$, we define their product $fg$ by

 $(fg)(x)=\sum_{x}f(x_{(1)})\cdot g(x_{(2)}),$

where we use the Sweedler notation

 $\Delta(x)=\sum_{x}x_{(1)}\otimes x_{(2)}$

for the comultiplication $\Delta$. To see that the convolution product is associative, suppose $f$, $g$, and $h$ are in $\mathrm{Hom}_{R}({C},{A})$. By applying the coassociativity of $\Delta$, we may write

 $((fg)h)(x)=\sum_{x}(f(x_{(1)})\cdot g(x_{(2)}))\cdot h(x_{(3)})$

and

 $(f(gh))(x)=\sum_{x}f(x_{(1)})\cdot(g(x_{(2)}))\cdot h(x_{(3)}).$

Since $A$ has an associative product, it follows that $(fg)h=f(gh)$.

In the foregoing, we have not assumed that $C$ is counitary or that $A$ is unitary. If $C$ is counitary with counit $\varepsilon\colon C\to R$ and $A$ is unitary with identity $1\colon R\to A$, then their composition   $1\circ\varepsilon\colon C\to A$ is the identity for the convolution product.

###### Example.

Let $C$ be a coassociative coalgebra over $R$. Then $R$ itself is an associative $R$-algebra. The algebra $\mathrm{Hom}_{R}({C},{R})$ is called the algebra dual to the coalgebra $C$.

We have seen that any coalgebra dualizes to give an algebra. One might expect that a similar construction could be performed on $\mathrm{Hom}_{R}({A},{R})$ to give a coalgebra dual to $A$. However, this is not the case. Thus coalgebras (based on “factoring”) are more fundamental than algebras (based on “multiplying”).

(The proof will be provided at a later stage).

Remark on Al/gebraic DualityMirror or tangled ‘duality’ of algebras and ‘gebras’:
An interesting twist to duality was provided in Fauser’s publications on al/gebras where mirror or tangled ‘duality’ has been defined for Grassman-Hopf al/gebras. Thus, an algebra not only has the usual reversed arrow dual coalgebra but a mirror (or tangled) gebra which is quite distinct from the coalgebra.

## References

Title dual of a coalgebra is an algebra, the DualOfACoalgebraIsAnAlgebraThe 2013-03-22 16:34:20 2013-03-22 16:34:20 mps (409) mps (409) 8 mps (409) Derivation msc 16W30 GrassmanHopfAlgebrasAndTheirDualCoAlgebras DualityInMathematics QuantumGroups dualities of algebraic structures