a(n) = (5^n - 1)/(2^(3 - (n mod 2))).

Terms

a(0) =0a(1) =1a(2) =3a(3) =31a(4) =78a(5) =781a(6) =1953a(7) =19531a(8) =48828a(9) =488281a(10) =1220703a(11) =12207031a(12) =30517578a(13) =305175781a(14) =762939453a(15) =7629394531a(16) =19073486328a(17) =190734863281a(18) =476837158203