On canonical antichains

An antichain A of a well-founded quasi-order Q is canonical if for every ideal F of Q , F has an infinite antichain if and only if F ∩ A is infinite. In this paper we characterize the obstructions to having a canonical antichain. As an application we show that, under the induced subgraph relation, the class of finite graphs does not have a canonical antichain. In contrast, this class does have a canonical antichain with respect to the subgraph relation.