%I M3959 #9 Nov 09 2018 16:23:01
%S 0,1,5,30,186,1164,7344,46732,299604,1932900,12537542,81705782,
%T 534663812,3511466838,23136724382,152888000934,1012925595468,
%U 6726766841438,44767880394634,298522284224824,1994187071224056,13343458013982572,89420609914983270,600088861692380354,4032377457462571694,27129080022839863318,182726605700552001998
%N Sum of squared spans of 2n-step polygons on square lattice.
%D N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
%H A. J. Guttmann and I. G. Enting, <a href="https://doi.org/10.1088/0305-4470/21/3/009">The size and number of rings on the square lattice</a>, J. Phys. A 21 (1988), L165-L172.
%Y Cf. A006772.
%K nonn
%O 1,3
%A _N. J. A. Sloane_
%E Name corrected, more terms from _Andrey Zabolotskiy_, Nov 09 2018