Reverse mathematics, well-quasi-orders, and Noetherian spaces

A quasi-order $Q$ induces two natural quasi-orders on $P(Q)$, but if $Q$ is a well-quasi-order, then these quasi-orders need not necessarily be well-quasi-orders. Nevertheless, Goubault-Larrecq showed that moving from a well-quasi-order $Q$ to the quasi-orders on $P(Q)$ preserves well-quasi-orderedness in a topological sense. Specifically, Goubault-Larrecq proved that the upper topologies of the induced quasi-orders on $P(Q)$ are Noetherian, which means that they contain no infinite strictly descending sequences of closed sets. We analyze various theorems of the form if $Q$ is a well-quasi-order then a certain topology on (a subset of) $P(Q)$ is Noetherian in the style of reverse mathematics, proving that these theorems are equivalent to ACA_0 over RCA_0. To state these theorems in RCA_0 we introduce a new framework for dealing with second-countable topological spaces.

Reverse Mathematics and initial intervals

In this paper we study the reverse mathematics of two theorems by Bonnet about partial orders. These results concern the structure and cardinality of the collection of the initial intervals. The first theorem states that a partial order has no infinite antichains if and only if its initial intervals are finite unions of ideals. The second one asserts that a countable partial order is scattered and does not contain infinite antichains if and only if it has countably many initial intervals. We show that the left to right directions of these theorems are equivalent to ACA_0 and ATR_0, respectively. On the other hand, the opposite directions are both provable in WKL_0, but not in RCA_0. We also prove the equivalence with ACA_0 of the following result of Erdos and Tarski: a partial order with no infinite strong antichains has no arbitrarily large finite strong antichains.

Linear extensions of partial orders and Reverse Mathematics

We introduce the notion of tau-like partial order, where tau is one of the linear order types omega, omega*, omega+omega*, and zeta. For example, being omega-like means that every element has finitely many predecessors, while being zeta-like means that every interval is finite. We consider statements of the form any tau-like partial order has a tau-like linear extension and any tau-like partial order is embeddable into tau (when tau is zeta this result appears to be new). Working in the framework of reverse mathematics, we show that these statements are equivalent either to BSigma^0_2 or to ACA_0 over the usual base system RCA_0.