274810802176domain: NAppears in sequencesNumbers of the form m*(2^k-1), where m = 2^(k-1)*(2^k-1) is a perfect number (A000396).at n=4A181711a(n) is the least k that is a multiple of d=A240991(n), with abundancy ratio sigma(k)/k equal to (sigma(d)+1)/d, or -1 if no such k exists.at n=31A387405