a(n) = T(n,2n-1), T given by A027023.

Terms

a(0) =1a(1) =3a(2) =5a(3) =11a(4) =25a(5) =59a(6) =145a(7) =367a(8) =949a(9) =2495a(10) =6645a(11) =17883a(12) =48541a(13) =132711a(14) =365073a(15) =1009647a(16) =2805365a(17) =7827167a(18) =21918997a(19) =61584891a(20) =173550677a(21) =490408623a(22) =1389206065a(23) =3944231887a(24) =11221911849a(25) =31989733339a(26) =91354992405a(27) =261322661051