Number of self-avoiding cycles of length 2n on the half-Manhattan lattice.

Terms

a(0) =1a(1) =0a(2) =1a(3) =1a(4) =2a(5) =6a(6) =17a(7) =49a(8) =156a(9) =520a(10) =1762a(11) =6110a(12) =21713a(13) =78469a(14) =287393a(15) =1065951a(16) =3998286a(17) =15140500a(18) =57817817a(19) =222478829a(20) =861952579a(21) =3360043843a(22) =13171307962