a(n) = 5^n + (1 - 4^n)/3.

Terms

a(0) =1a(1) =4a(2) =20a(3) =104a(4) =540a(5) =2784a(6) =14260a(7) =72664a(8) =368780a(9) =1865744a(10) =9416100a(11) =47430024a(12) =238548220a(13) =1198333504a(14) =6014037140a(15) =30159664184a(16) =151156234860a(17) =757212830064