a(n) = 2*4^(n-1) - (3n-1)/(2n+2)*C(2n,n).

Terms

a(0) =1a(1) =3a(2) =12a(3) =51a(4) =218a(5) =926a(6) =3902a(7) =16323a(8) =67866a(9) =280746a(10) =1156576a(11) =4748398a(12) =19439332a(13) =79391708a(14) =323584322a(15) =1316578403a(16) =5348814842a(17) =21702312818a(18) =87955584152a(19) =356114261498