Orbits of toric promotion on bridge sums

Abstract: In 2023, Defant introduced toric promotion as a cyclic analogue of Schützenberger's well known promotion operator. Toric promotion is defined by a choice of simple graph $G$ and acts on the labeling of $G$ by a series of involutions. Defant described the orbit length of toric promotion on trees and showed that it does not depend on the initial labeling; we prove an analogous result for complete graphs. A natural question is how toric promotion behaves under certain graph operations. In the main results of this article, we analyze the orbits of toric promotion under the bridge sum graph operation, which joins two graphs by adding an edge between a vertex of each graph. We show that the orbit length of toric promotion on any graph constructed via a bridge sum of a tree or a complete graph with a simple graph does not depend on the restriction of the initial labeling to the tree or complete subgraph. Additionally, we describe the orbit lengths of toric promotion on the bridge sums of two complete graphs and the bridge sums of a tree with a complete graph, and show that they do not depend on the initial labeling. Finally, we describe the orbit length of toric promotion on the corona product of a complete graph with any tree, and show that it does not depend on the initial labeling.