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

Terms

a(0) =1a(1) =3a(2) =21a(3) =87a(4) =489a(5) =2283a(6) =11901a(7) =58047a(8) =294609a(9) =1459923a(10) =7338981a(11) =36576807a(12) =183238329a(13) =915128763a(14) =4578832461a(15) =22884596367a(16) =114451679649a(17) =572172304803