a(n) = 4*a(n-2)+1 with initial terms 1,3,7.

Terms

a(0) =1a(1) =3a(2) =7a(3) =13a(4) =29a(5) =53a(6) =117a(7) =213a(8) =469a(9) =853a(10) =1877a(11) =3413a(12) =7509a(13) =13653a(14) =30037a(15) =54613a(16) =120149a(17) =218453a(18) =480597a(19) =873813a(20) =1922389a(21) =3495253a(22) =7689557a(23) =13981013a(24) =30758229a(25) =55924053a(26) =123032917a(27) =223696213a(28) =492131669a(29) =894784853