121400domain: NAppears in sequencesNumber of nontrivial compositions of differential operations and directional derivative of the n-th order on the space R^9.at n=25A187107Integers x such that there exist four integers 0<y<=z<=t<=w such that sigma(x)^5 = x^5 + y^5 + z^5 + t^5 + w^5.at n=13A386672