3504816domain: NAppears in sequencesTheta series of D_21 lattice.at n=3A022052Theta series of D*_21 lattice.at n=24A022074a(n) = (4/45)*n*(n - 2)*(n - 1)*(n^3 - 12*n^2 + 47*n - 15).at n=21A319577Number x such that sigma(x) = Sum_{i=1..k} {sigma(x/p_i)}, where p_i are the k prime factors of x.at n=21A324711