Number of compositions of 2n where the multiplicity of the first part equals n.

Terms

a(0) =1a(1) =1a(2) =3a(3) =4a(4) =15a(5) =36a(6) =126a(7) =372a(8) =1239a(9) =3910a(10) =12848a(11) =41581a(12) =136578a(13) =447188a(14) =1473342a(15) =4855704a(16) =16053831a(17) =53138244a(18) =176233968a(19) =585202262a(20) =1945964080a(21) =6478043121a(22) =21588979877a(23) =72016891509a(24) =240452892570a(25) =803489258286