a(n) = T(n,n+1), array T as in A047130.

Terms

a(0) =1a(1) =2a(2) =5a(3) =14a(4) =56a(5) =188a(6) =639a(7) =2206a(8) =8485a(9) =31278a(10) =114960a(11) =423612a(12) =1621969a(13) =6160764a(14) =23359289a(15) =88603820a(16) =341015446a(17) =1312635338a(18) =5052259362a(19) =19452272144a(20) =75341562629a(21) =292245458188