Number of polygons of length 4n on L-lattice.

Terms

a(0) =1a(1) =0a(2) =1a(3) =2a(4) =9a(5) =36a(6) =154a(7) =684a(8) =3128a(9) =14666a(10) =70258a(11) =342766a(12) =1698625a(13) =8532410a(14) =43368153a(15) =222729492a(16) =1154455161a(17) =6033034032a(18) =31760434883a(19) =168311948410a(20) =897327111964