a(n) = 4*a(n-2) + 1 with a(1) = 0, a(2) = 3.

Terms

a(0) =0a(1) =3a(2) =1a(3) =13a(4) =5a(5) =53a(6) =21a(7) =213a(8) =85a(9) =853a(10) =341a(11) =3413a(12) =1365a(13) =13653a(14) =5461a(15) =54613a(16) =21845a(17) =218453a(18) =87381a(19) =873813a(20) =349525a(21) =3495253a(22) =1398101a(23) =13981013a(24) =5592405a(25) =55924053a(26) =22369621a(27) =223696213a(28) =89478485a(29) =894784853