a(n) = T(n, 2*n-7), T given by A027926.

Terms

a(0) =1a(1) =3a(2) =8a(3) =21a(4) =54a(5) =133a(6) =309a(7) =674a(8) =1383a(9) =2683a(10) =4950a(11) =8735a(12) =14820a(13) =24285a(14) =38587a(15) =59652a(16) =89981a(17) =132771a(18) =192052a(19) =272841a(20) =381314a(21) =524997a(22) =712977a(23) =956134a(24) =1267395a(25) =1662011a(26) =2157858a(27) =2775763a(28) =3539856a(29) =4477949