a(n) = ((1 + y)^n - (1 - y)^n)/y with y = sqrt(8).

Terms

a(0) =0a(1) =2a(2) =4a(3) =22a(4) =72a(5) =298a(6) =1100a(7) =4286a(8) =16272a(9) =62546a(10) =238996a(11) =915814a(12) =3504600a(13) =13419898a(14) =51371996a(15) =196683278a(16) =752970528a(17) =2882724002a(18) =11036241700a(19) =42251551414a(20) =161756794728a(21) =619274449354