a(n) = 3^n - 5.

Terms

a(0) =-4a(1) =-2a(2) =4a(3) =22a(4) =76a(5) =238a(6) =724a(7) =2182a(8) =6556a(9) =19678a(10) =59044a(11) =177142a(12) =531436a(13) =1594318a(14) =4782964a(15) =14348902a(16) =43046716a(17) =129140158a(18) =387420484a(19) =1162261462a(20) =3486784396a(21) =10460353198a(22) =31381059604a(23) =94143178822a(24) =282429536476a(25) =847288609438