a(n) = ceiling(a(n-1)*4/3), with a(1) = 1.

Terms

a(0) =1a(1) =2a(2) =3a(3) =4a(4) =6a(5) =8a(6) =11a(7) =15a(8) =20a(9) =27a(10) =36a(11) =48a(12) =64a(13) =86a(14) =115a(15) =154a(16) =206a(17) =275a(18) =367a(19) =490a(20) =654a(21) =872a(22) =1163a(23) =1551a(24) =2068a(25) =2758a(26) =3678a(27) =4904a(28) =6539a(29) =8719