a(n) = (11*3^n+1)/2.

Terms

a(0) =6a(1) =17a(2) =50a(3) =149a(4) =446a(5) =1337a(6) =4010a(7) =12029a(8) =36086a(9) =108257a(10) =324770a(11) =974309a(12) =2922926a(13) =8768777a(14) =26306330a(15) =78918989a(16) =236756966a(17) =710270897a(18) =2130812690a(19) =6392438069a(20) =19177314206a(21) =57531942617a(22) =172595827850a(23) =517787483549