2247000domain: NAppears in sequences(1/7) times the sum of the elements of all subsets of [n] whose sum is divisible by seven.at n=20A309299a(1) = 2, a(n) = k - 1, where k is the least number greater than a(n-1) such that rad(k) | a(n-1), where rad(n) = A007947(n).at n=25A365413