A000458 a(0) = a(1) = 1; thereafter a(n) = sigma(a(n-1)) + sigma(a(n-2)).

1, 1, 2, 4, 10, 25, 49, 88, 237, 500, 1412, 3570, 12846, 36072, 126504, 493920, 2358720, 12292224, 49984224, 171237888, 642804078, 1980490350, 6380883000, 27032104440, 117961599600, 555861355920, 2623445776224