The finite Rat-splitting for coalgebras

Abstract

Let $C$ be a coalgebra. We investigate the problem of when the rational part of every finitely generated $C^*$-module $M$ is a direct summand $M$. We show that such a coalgebra must have at most countable dimension, $C$ must be artinian as right $C^*$-module and injective as left $C^*$-module. Also in this case $C^*$ is a left Noetherian ring. Following the classic example of the divided power coalgebra where this property holds, we investigate a more general type of coalgebras, the chain coalgebras, which are coalgebras whose lattice of left (or equivalently, right, two-sided) coideals form a chain. We show that this is a left-right symmetric concept and that these coalgebras have the above stated splitting property. Moreover, we show that this type of coalgebras are the only infinite dimensional colocal coalgebras for which the rational part of every finitely generated left $C^*$-module $M$ splits off in $M$, so this property is also left-right symmetric and characterizes the chain coalgebras among the colocal coalgebras.