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

Terms

a(0) =1a(1) =7a(2) =37a(3) =179a(4) =826a(5) =3703a(6) =16283a(7) =70619a(8) =303121a(9) =1290682a(10) =5460511a(11) =22981019a(12) =96296552a(13) =402024497a(14) =1673116072a(15) =6944105579a(16) =28752345362a(17) =118801061059a(18) =489959398840