Number of 2n-step polygons on honeycomb.

Terms

a(0) =1a(1) =0a(2) =0a(3) =6a(4) =0a(5) =30a(6) =24a(7) =168a(8) =288a(9) =1170a(10) =2760a(11) =9504a(12) =25776a(13) =84006a(14) =244104a(15) =779970a(16) =2356800a(17) =7487310a(18) =23167656a(19) =73639326a(20) =231348000a(21) =737718534a(22) =2341772136a(23) =7498417956a(24) =23982456384a(25) =77119604400a(26) =248088123192a(27) =800978437674