141085domain: NAppears in sequencesa(n) = Sum_{k=1..n-1} k*sigma(k)*sigma(n-k).at n=28A0004414-dimensional figurate numbers: a(n) = (5*n-1)*binomial(n+2,3)/4.at n=28A002418Starting values k of Collatz orbits that achieve a new minimum of Product_{j == 4 mod 6 in "3x+1" orbit of k} (j-1)/j.at n=21A391017