Number of neighbor-avoiding polygons of perimeter 2n on square lattice.

Terms

a(0) =0a(1) =1a(2) =0a(3) =1a(4) =2a(5) =9a(6) =36a(7) =154a(8) =668a(9) =2932a(10) =13016a(11) =58364a(12) =264208a(13) =1206818a(14) =5558724a(15) =25803509a(16) =120638466a(17) =567732133a(18) =2687937916a(19) =12796823923a(20) =61235363802a(21) =294407424869