a(1) = 1; a(n+1) = Sum_{d|n} sigma(n/d)*a(d), where sigma = sum of divisors (A000203).

Terms

a(0) =1a(1) =1a(2) =4a(3) =8a(4) =18a(5) =24a(6) =52a(7) =60a(8) =106a(9) =135a(10) =213a(11) =225a(12) =397a(13) =411a(14) =599a(15) =719a(16) =1001a(17) =1019a(18) =1533a(19) =1553a(20) =2192a(21) =2464a(22) =3151a(23) =3175a(24) =4502a(25) =4641a(26) =5888a(27) =6404a(28) =8145a(29) =8175