A simple grammar: partial sums of A052870.

Terms

a(0) =0a(1) =1a(2) =2a(3) =4a(4) =10a(5) =25a(6) =69a(7) =197a(8) =583a(9) =1762a(10) =5441a(11) =17042a(12) =54072a(13) =173334a(14) =560659a(15) =1827306a(16) =5995570a(17) =19787135a(18) =65643226a(19) =218777532a(20) =732181107a(21) =2459576149a(22) =8290442750a(23) =28031056619a(24) =95045477945a(25) =323112137130