a(n) = ceiling of Gamma(n + 2/9)/Gamma(2/9).

Terms

a(0) =1a(1) =1a(2) =1a(3) =1a(4) =2a(5) =9a(6) =43a(7) =267a(8) =1928a(9) =15845a(10) =146123a(11) =1493701a(12) =16762640a(13) =204876709a(14) =2708925369a(15) =38526938569a(16) =586465620431