Erroneous duplicate of A000170, see comments.

Terms

a(0) =1a(1) =0a(2) =0a(3) =2a(4) =10a(5) =4a(6) =40a(7) =92a(8) =352a(9) =724a(10) =2680a(11) =14200a(12) =73712a(13) =365596a(14) =2279184a(15) =14772512a(16) =95815104a(17) =666090624a(18) =4968057848a(19) =39029188884a(20) =314666222712