Partial sums of A006864.

Terms

a(0) =0a(1) =1a(2) =3a(3) =9a(4) =23a(5) =60a(6) =152a(7) =388a(8) =984a(9) =2501a(10) =6347a(11) =16117a(12) =40911a(13) =103864a(14) =263664a(15) =669352a(16) =1699216a(17) =4313673a(18) =10950739a(19) =27799745a(20) =70572839a(21) =179157364a(22) =454811656a(23) =1154592108a(24) =2931065640a(25) =7440849549a(26) =18889457819a(27) =47953075565