Counting orbits under Kreweras complementation

Abstract

The Kreweras complementation map is an anti-isomorphism on the lattice of noncrossing partitions. We consider an analogous operation for plane trees motivated by the molecular biology problem of RNA folding. In this context, we explicitly count the orbits of Kreweras' map according to their length as the number of appropriate symmetry classes of trees in the plane. These enumeration results are consolidated into a single implicit formula under the cyclic sieving phenomenon.