a(n) = floor(9^n/2^n).

Terms

a(0) =1a(1) =4a(2) =20a(3) =91a(4) =410a(5) =1845a(6) =8303a(7) =37366a(8) =168151a(9) =756680a(10) =3405062a(11) =15322783a(12) =68952523a(13) =310286355a(14) =1396288601a(15) =6283298708a(16) =28274844190a(17) =127236798856a(18) =572565594852