a(n) = 3*a(n-1) + 2*a(n-3), with a(0)=1, a(1)=3.

Terms

a(0) =1a(1) =3a(2) =9a(3) =29a(4) =93a(5) =297a(6) =949a(7) =3033a(8) =9693a(9) =30977a(10) =98997a(11) =316377a(12) =1011085a(13) =3231249a(14) =10326501a(15) =33001673a(16) =105467517a(17) =337055553a(18) =1077170005a(19) =3442445049a(20) =11001446253a(21) =35158678769a(22) =112360926405a(23) =359085671721