We consider the posets of equivalence relations on finite sets under the standard embedding ordering and under the consecutive embedding ordering. In the latter case, the relations are also assumed to have an underlying linear order, which governs consecutive embeddings. For each poset we ask the well quasi-order and atomicity decidability questions: Given finitely many equivalence relations $\rho_1,\dots,\rho_k$, is the downward closed set Av$(\rho_1,\dots,\rho_k)$ consisting of all equivalence relations which do not contain any of $\rho_1,\dots,\rho_k$: (a) well-quasi-ordered, meaning that it contains no infinite antichains? and (b) atomic, meaning that it is not a union of two proper downward closed subsets, or, equivalently, that it satisfies the joint embedding property?
翻译:我们考虑在标准嵌入序和连续嵌入序下有限集上等价关系的偏序集。在后一种情形中,假定这些关系具有一个底层线性序,该序控制连续嵌入。针对每个偏序集,我们提出良拟序与原子性的可判定性问题:给定有限多个等价关系 $\rho_1,\dots,\rho_k$,由不包含任何 $\rho_1,\dots,\rho_k$ 的所有等价关系构成的下向闭集 Av$(\rho_1,\dots,\rho_k)$ 是否 (a) 是良拟序的,即它不包含无限反链?以及 (b) 是原子的,即它不能表示为两个真下向闭子集的并集,或等价地,它满足联合嵌入性质?