Partial sums of A006820.

Terms

a(0) =1a(1) =1a(2) =1a(3) =1a(4) =1a(5) =2a(6) =3a(7) =5a(8) =11a(9) =27a(10) =86a(11) =351a(12) =1895a(13) =12673a(14) =100841a(15) =906332a(16) =8943750a(17) =95165384a(18) =1081035906a(19) =13027523553a(20) =165835586734