a(n) = (11*2^n - 2*(-1)^n)/3 for n >= 0.

Terms

a(0) =3a(1) =8a(2) =14a(3) =30a(4) =58a(5) =118a(6) =234a(7) =470a(8) =938a(9) =1878a(10) =3754a(11) =7510a(12) =15018a(13) =30038a(14) =60074a(15) =120150a(16) =240298a(17) =480598a(18) =961194a(19) =1922390a(20) =3844778a(21) =7689558a(22) =15379114a(23) =30758230a(24) =61516458a(25) =123032918a(26) =246065834a(27) =492131670a(28) =984263338a(29) =1968526678