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

Terms

a(0) =3a(1) =1a(2) =2a(3) =7a(4) =15a(5) =32a(6) =71a(7) =157a(8) =346a(9) =763a(10) =1683a(11) =3712a(12) =8187a(13) =18057a(14) =39826a(15) =87839a(16) =193735a(17) =427296a(18) =942431a(19) =2078597a(20) =4584490a(21) =10111411a(22) =22301419a(23) =49187328a(24) =108486067a(25) =239273553a(26) =527734434a(27) =1163954935a(28) =2567183423a(29) =5662101280