a(n) = 4*a(n-1) + 1 with a(0) = 3.

Terms

a(0) =3a(1) =13a(2) =53a(3) =213a(4) =853a(5) =3413a(6) =13653a(7) =54613a(8) =218453a(9) =873813a(10) =3495253a(11) =13981013a(12) =55924053a(13) =223696213a(14) =894784853a(15) =3579139413a(16) =14316557653a(17) =57266230613a(18) =229064922453a(19) =916259689813