a(n) = T(n, 2*n-5), T given by A027926.

Terms

a(0) =1a(1) =3a(2) =8a(3) =20a(4) =46a(5) =97a(6) =189a(7) =344a(8) =591a(9) =967a(10) =1518a(11) =2300a(12) =3380a(13) =4837a(14) =6763a(15) =9264a(16) =12461a(17) =16491a(18) =21508a(19) =27684a(20) =35210a(21) =44297a(22) =55177a(23) =68104a(24) =83355a(25) =101231a(26) =122058a(27) =146188a(28) =174000a(29) =205901