a(n) = 7*a(n-1) - 14*a(n-2) + 7*a(n-3), a(0)=0, a(1)=1, a(2)=5.

Terms

a(0) =0a(1) =1a(2) =5a(3) =21a(4) =84a(5) =329a(6) =1274a(7) =4900a(8) =18767a(9) =71687a(10) =273371a(11) =1041348a(12) =3964051a(13) =15083082a(14) =57374296a(15) =218205281a(16) =829778397a(17) =3155194917a(18) =11996903828a(19) =45614046737a(20) =173428037986a(21) =659377938380