a(n) = 4*a(n-1) + 1, a(1) = 15.

Terms

a(0) =15a(1) =61a(2) =245a(3) =981a(4) =3925a(5) =15701a(6) =62805a(7) =251221a(8) =1004885a(9) =4019541a(10) =16078165a(11) =64312661a(12) =257250645a(13) =1029002581a(14) =4116010325a(15) =16464041301a(16) =65856165205a(17) =263424660821