On the Number of Saturated and Optimal Extended 2-Regular Simple Stacks in the Nussinov-Jacobson Energy Model.

It is known that both RNA secondary structure and protein contact map can be presented using combinatorial diagrams, the combinatorial enumeration and related problems of which have been studied extensively. Motivated by previous enumeration works on saturated RNA secondary structures and extended stack structures of protein contact maps, we are interested in the enumeration problems of saturated and optimal extended stacks in the Nussinov-Jacobson energy model, in which each base pair contributes energy -1. Then optimal structures are those with most arcs, and locally optimal structures are exactly the saturated structures, in which no more arcs can be added without violating the structure definition. For saturated extended 2-regular simple stacks, whose degree configuration is related to the protein fold in two-dimensional honeycomb lattice, we obtain generating function equation and asymptotic formula for its number. Moreover, an explicit formula for the number of optimal extended 2-regular simple stacks is also obtained.