Number of three-choice polygons by half-perimeter.

Terms

a(0) =4a(1) =12a(2) =42a(3) =152a(4) =562a(5) =2108a(6) =7986a(7) =30476a(8) =116950a(9) =450756a(10) =1743470a(11) =6763152a(12) =26299104a(13) =102478208a(14) =400032822a(15) =1563985172a(16) =6122945694a(17) =23999991452a(18) =94173327894a(19) =369881810616