a(n) = Sum_{k=0..n} (k+1) * A026747(n, n-k).

Terms

a(0) =1a(1) =3a(2) =10a(3) =25a(4) =73a(5) =170a(6) =468a(7) =1050a(8) =2799a(9) =6138a(10) =16043a(11) =34644a(12) =89390a(13) =190913a(14) =488285a(15) =1034228a(16) =2628861a(17) =5532322a(18) =14000177a(19) =29311171a(20) =73936801a(21) =154145390a(22) =387911301a(23) =805899231a(24) =2024537328a(25) =4193625311a(26) =10521409964a(27) =21738996250