a(0) = a(1) = 1; thereafter a(n) = sigma(a(n-1)) + sigma(a(n-2)).

Terms

a(0) =1a(1) =1a(2) =2a(3) =4a(4) =10a(5) =25a(6) =49a(7) =88a(8) =237a(9) =500a(10) =1412a(11) =3570a(12) =12846a(13) =36072a(14) =126504a(15) =493920a(16) =2358720a(17) =12292224a(18) =49984224a(19) =171237888a(20) =642804078a(21) =1980490350a(22) =6380883000a(23) =27032104440a(24) =117961599600a(25) =555861355920