a(n) = Sum_{0<=j<=i, 0<=i<=n} A027907(i, j).

Terms

a(0) =1a(1) =3a(2) =9a(3) =26a(4) =76a(5) =223a(6) =658a(7) =1948a(8) =5782a(9) =17193a(10) =51194a(11) =152594a(12) =455209a(13) =1358841a(14) =4058439a(15) =12126696a(16) =36248370a(17) =108385917a(18) =324172566a(19) =969801726a(20) =2901883611a(21) =8684735577a(22) =25995833145a(23) =77824036620a(24) =233012973051a(25) =697745695923