%I #6 Mar 30 2012 16:48:51
%S 1,24,342,3804,36802,326540,2735308,22006264,171912401,1313614776,
%T 9868281330,73150450248,536495354621,3900954906412,28164962681150,
%U 202166363020920,1444068530799525,10272599729628876
%N Number of polygons of length 2n with one hole on square lattice (not allowing rotations).
%H I. Jensen, <a href="/A056634/b056634.txt">Table of n, a(n) for n = 8..41</a> (from link below)
%H I. Jensen, <a href="http://www.ms.unimelb.edu.au/~iwan/polygons/series/square.perim.1hole.ser">More terms</a>
%Y Cf. A002931.
%K nonn
%O 8,2
%A _N. J. A. Sloane_, Aug 30 2000