a(n) = A027113(n, 2n-1).

Terms

a(0) =1a(1) =4a(2) =9a(3) =23a(4) =59a(5) =154a(6) =407a(7) =1088a(8) =2935a(9) =7980a(10) =21843a(11) =60139a(12) =166425a(13) =462638a(14) =1291245a(15) =3616923a(16) =10164335a(17) =28648092a(18) =80960915a(19) =229360649a(20) =651237509a(21) =1852936294a(22) =5282196109a(23) =15084862158a(24) =43150636493a(25) =123624304356a(26) =354689105733