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

Terms

a(0) =1a(1) =3a(2) =3a(3) =5a(4) =13a(5) =27a(6) =51a(7) =101a(8) =205a(9) =411a(10) =819a(11) =1637a(12) =3277a(13) =6555a(14) =13107a(15) =26213a(16) =52429a(17) =104859a(18) =209715a(19) =419429a(20) =838861a(21) =1677723a(22) =3355443a(23) =6710885a(24) =13421773a(25) =26843547a(26) =53687091a(27) =107374181a(28) =214748365a(29) =429496731