126854domain: NAppears in sequencesa_n = - sum_{i=1..n-1} C(i+1, n-i) (-1)^(n-i) a_i.at n=7A028380a(n) is the least number k such that there is a set of n proper divisors of k whose sum is k, and no set of fewer than n proper divisors of k whose sum is k.at n=19A389712