The order of birational rowmotion

Various authors have studied a natural operation (under various names) on the order ideals (equivalently antichains) of a finite poset, here called rowmotion. For certain posets of interest, the order of this map is much smaller than one would naively expect, and the orbits exhibit unexpected properties. In very recent work (inspired by discussions with Berenstein) Einstein and Propp describe how rowmotion can be generalized: first to the piecewise-linear setting of order polytopes, then via detropicalization to the birational setting. In the latter setting, it is no longer a priori clear even that birational rowmotion has finite order, and for many posets the order is infinite. However, we are able to show that birational rowmotion has the same order, p + q, for the poset P = [p] × [q] (product of two chains), as ordinary rowmotion. We also show that birational (hence ordinary) rowmotion has finite order for some other classes of posets, e.g., the upper, lower, right and left halves of the poset above, and trees having all leaves on the same level. Our methods are based on those used by Volkov to resolve the type AA (rectangular) Zamolodchikov Periodicity Conjecture. Résumé. Une opération naturelle sur les idéaux d’ordre (ou, de façon équivalente, sur les antichaı̂nes) d’un ensemble partielle-ment ordonne ́ fini, appelée ici rowmotion, a éte ́ étudiée par divers auteurs. Pour certains ensembles partiellement ordonnés particulièrement intéressants, l’ordre de cette application rowmotion est beaucoup plus petit que ce a ̀ quoi on peut s’attendre naı̈vement, et ses orbites ont des propriétés inattendues. Dans un travail très récent (inspire ́ par des conversations avec Berenstein), Einstein et Propp ont décrit comment le rowmotion peut être généralisé: d’abord au contexte linéaire par morceaux des polytopes d’ordre, ensuite au contexte birationnel par détropicalisation