a(n) = 4*a(n-1) + 4*a(n-2), n>2, a(0)=1, a(1)=3, a(2)=15.

Terms

a(0) =1a(1) =3a(2) =15a(3) =72a(4) =348a(5) =1680a(6) =8112a(7) =39168a(8) =189120a(9) =913152a(10) =4409088a(11) =21288960a(12) =102792192a(13) =496324608a(14) =2396467200a(15) =11571167232a(16) =55870537728a(17) =269766819840