Partial sums of A000084.

Terms

a(0) =0a(1) =1a(2) =3a(3) =7a(4) =17a(5) =41a(6) =107a(7) =287a(8) =809a(9) =2341a(10) =6965a(11) =21101a(12) =65031a(13) =202939a(14) =640441a(15) =2039509a(16) =6546861a(17) =21158437a(18) =68791923a(19) =224839127a(20) =738316629a(21) =2434622357a(22) =8058616301a(23) =26765349429a(24) =89173526191a(25) =297942766331a(26) =998072479961