787127domain: NAppears in sequencesNumbers k such that sum of divisors of k^2 is a square.at n=35A008847Odd numbers n such that sigma(sigma(n^2)) is odd.at n=10A234641T(n,k) = 1/k! * Sum_{i=0..k} (-1)^(k-i) *C(k,i) * A258309(n,i); triangle T(n,k), n>=0, 0<=k<=floor(n/2), read by rows.at n=38A258310