a(n) = T(2*n+1,n), with T given by A027907.

Terms

a(0) =1a(1) =3a(2) =15a(3) =77a(4) =414a(5) =2277a(6) =12727a(7) =71955a(8) =410346a(9) =2355962a(10) =13599915a(11) =78855339a(12) =458917850a(13) =2679183405a(14) =15683407785a(15) =92022516525a(16) =541050073146