a(n) = s(n+3)/2, where s is A024963.

Terms

a(0) =3a(1) =9a(2) =29a(3) =93a(4) =297a(5) =949a(6) =3195a(7) =10701a(8) =35675a(9) =118761a(10) =412065a(11) =1420297a(12) =4867545a(13) =16663185a(14) =58759435a(15) =206130225a(16) =719682435a(17) =2508537225a(18) =8956415877a(19) =31817261421a(20) =112516001801a(21) =397401991437