Duplicate of A028270.

Terms

a(0) =1a(1) =3a(2) =8a(3) =26a(4) =90a(5) =322a(6) =1176a(7) =4356a(8) =16302a(9) =61490a(10) =233376a(11) =890188a(12) =3409588a(13) =13104756