a(n) = 4*a(n-1) + 4*a(n-2) - a(n-3); a(0)=1, a(1)=1, a(2)=7.

Terms

a(0) =1a(1) =1a(2) =7a(3) =31a(4) =151a(5) =721a(6) =3457a(7) =16561a(8) =79351a(9) =380191a(10) =1821607a(11) =8727841a(12) =41817601a(13) =200360161a(14) =959983207a(15) =4599555871a(16) =22037796151a(17) =105589424881a(18) =505909328257