Linear extensions of partial orders and Reverse Mathematics

Emanuele Frittaion, Alberto Marcone

Published 2012-03-23, updated 2012-04-15Version 2

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 B\Sigma^0_2 or to ACA_0 over the usual base system RCA_0.