A Procedure for Improving the Upper Bound for the Number of n-Ominoes
D. A. Klarner, Ronald L. Rivest · Canadian Journal of Mathematics · 1973
We begin with some definitions and a formulation of the problem treated in subsequent sections. Also included in this section is a brief indication of some of the known results dealing with the n-omino enumeration problem. Some of what follows together with more details may be found in [3] or [4].