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

Terms

a(0) =-1a(1) =17a(2) =11a(3) =113a(4) =179a(5) =857a(6) =1931a(7) =7073a(8) =18659a(9) =61097a(10) =173051a(11) =539633a(12) =1577939a(13) =4815737a(14) =14283371a(15) =43177793a(16) =128878019a(17) =387944777a(18) =1161212891a(19) =3488881553a(20) =10456158899a(21) =31389448217a(22) =94126401611a(23) =282463090913a(24) =847221500579