a(n) = (1/2)*A291383(n).

Terms

a(0) =1a(1) =4a(2) =14a(3) =49a(4) =172a(5) =604a(6) =2120a(7) =7442a(8) =26124a(9) =91704a(10) =321912a(11) =1130020a(12) =3966752a(13) =13924640a(14) =48880192a(15) =171585992a(16) =602324816a(17) =2114363648a(18) =7422130912a(19) =26054187664a(20) =91459003200a(21) =321052008000