Directed and multi-directed animals on the king's lattice

This article introduces a new, simple solvable lattice for directed animals: the directed king’s lattice, or square lattice with next nearest neighbor bonds and preferred directions {W, NW, N, NE, E}. We show that the directed animals in this lattice have an algebraic generating function linked to the Schr"oder numbers and belong to the same universality class as the ones in the square and triangular lattices. We also define multi-directed animals in the king’s lattice, which form a superclass of directed animals. We compute their generating function and show that it is not D-finite. Finally, we propose efficient random sampling algorithms for our animals.