a(n) = 2*A000984(n) - (n+1).

Terms

a(0) =1a(1) =2a(2) =9a(3) =36a(4) =135a(5) =498a(6) =1841a(7) =6856a(8) =25731a(9) =97230a(10) =369501a(11) =1410852a(12) =5408299a(13) =20801186a(14) =80233185a(15) =310235024a(16) =1202160763a(17) =4667212422a(18) =18150270581a(19) =70690527580a(20) =275693057619