a(n) = floor((3^n)/(2*n - 1)).

Terms

a(0) =3a(1) =3a(2) =5a(3) =11a(4) =27a(5) =66a(6) =168a(7) =437a(8) =1157a(9) =3107a(10) =8435a(11) =23106a(12) =63772a(13) =177147a(14) =494789a(15) =1388603a(16) =3913338a(17) =11069156a(18) =31412472a(19) =89404728a(20) =255130565a(21) =729792083a(22) =2092070640a(23) =6009139074a(24) =17291604274a(25) =49840506437a(26) =143879197829a(27) =415941680999