a(n) = (8-5*4^n)/3.

Terms

a(0) =1a(1) =-4a(2) =-24a(3) =-104a(4) =-424a(5) =-1704a(6) =-6824a(7) =-27304a(8) =-109224a(9) =-436904a(10) =-1747624a(11) =-6990504a(12) =-27962024a(13) =-111848104a(14) =-447392424a(15) =-1789569704a(16) =-7158278824a(17) =-28633115304a(18) =-114532461224a(19) =-458129844904