Bisection of A000081 (even part).

Terms

a(0) =0a(1) =1a(2) =4a(3) =20a(4) =115a(5) =719a(6) =4766a(7) =32973a(8) =235381a(9) =1721159a(10) =12826228a(11) =97055181a(12) =743724984a(13) =5759636510a(14) =45007066269a(15) =354426847597