a(n) = (1^n + (-2)^n + 4^n)/3.

Terms

a(0) =1a(1) =1a(2) =7a(3) =19a(4) =91a(5) =331a(6) =1387a(7) =5419a(8) =21931a(9) =87211a(10) =349867a(11) =1397419a(12) =5593771a(13) =22366891a(14) =89483947a(15) =357903019a(16) =1431677611a(17) =5726579371a(18) =22906579627a(19) =91625794219a(20) =366504225451