a(n) = Sum_{k=1..n} binomial(2*n, n-k) * sigma_2(k).

Terms

a(0) =1a(1) =9a(2) =55a(3) =297a(4) =1496a(5) =7215a(6) =33783a(7) =154825a(8) =698077a(9) =3107424a(10) =13690161a(11) =59802471a(12) =259377080a(13) =1118176887a(14) =4795381640a(15) =20472223529a(16) =87051685546a(17) =368857919085