a(n) = A027082(n, 2n-1).

Terms

a(0) =1a(1) =3a(2) =6a(3) =16a(4) =41a(5) =107a(6) =283a(7) =757a(8) =2043a(9) =5557a(10) =15216a(11) =41906a(12) =115999a(13) =322537a(14) =900406a(15) =2522616a(16) =7090307a(17) =19987069a(18) =56492426a(19) =160062636a(20) =454529595a(21) =1293394325a(22) =3687475479a(23) =10531663241a(24) =30128741717a(25) =86324342567a(26) =247691217352