a(n) = (5*3^n-3)/2.

Terms

a(0) =1a(1) =6a(2) =21a(3) =66a(4) =201a(5) =606a(6) =1821a(7) =5466a(8) =16401a(9) =49206a(10) =147621a(11) =442866a(12) =1328601a(13) =3985806a(14) =11957421a(15) =35872266a(16) =107616801a(17) =322850406a(18) =968551221a(19) =2905653666a(20) =8716961001a(21) =26150883006a(22) =78452649021a(23) =235357947066a(24) =706073841201