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

Terms

a(0) =1a(1) =2a(2) =5a(3) =14a(4) =39a(5) =108a(6) =299a(7) =828a(8) =2293a(9) =6350a(10) =17585a(11) =48698a(12) =134859a(13) =373464a(14) =1034231a(15) =2864088a(16) =7931497a(17) =21964634a(18) =60826493a(19) =168446342a(20) =466477167a(21) =1291811652a(22) =3577404131a(23) =9906877908a(24) =27435041245a(25) =75975649958a(26) =210398786537