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%I #14 Apr 18 2021 14:03:51
%S 1,4,26,240,2918,44424,816730,17652320
%N Number of lattice 2n-gons with edges on distinct lines.
%C a(n) is the number of self-avoiding polygons on the n X n square lattice having 2n edges (arbitrary perimeter) with an edge on each of the n distinct horizontal and n distinct vertical lines in the lattice.
%H StackExchange, <a href="http://math.stackexchange.com/questions/35211">Number of cycles in a grid such that each cycle traverse all the lines</a>
%e a(4)=26: 4 symmetries of the Z pentomino; 4 rotations of the step (123 bargraph) hexomino; 8 symmetries of each of the 132 and 213 bargraph hexominoes; and 2 rotations of the heptomino that consists of a 3x3 square with opposite corner cells removed.
%K nonn,more
%O 2,2
%A _David Bevan_, Jan 08 2012
%E a(8) and a(9) from _Francisco Mota_, Jun 06 2013
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