A141479(n+1)/3.

Terms

a(0) =1a(1) =1a(2) =1a(3) =2a(4) =6a(5) =21a(6) =91a(7) =462a(8) =2646a(9) =16841a(10) =117931a(11) =900922a(12) =7456086a(13) =66453661a(14) =634585771a(15) =6463837382a(16) =69955114326a(17) =801626558481