188860domain: NAppears in sequencesNumbers n such that sigma(n) = 8*phi(n).at n=26A104901a(n) is the first pentagonal number that is nontrivially the sum of two pentagonal numbers of the type P(p) + P(p+n) (we always have P(k) = P(0) + P(k)).at n=12A133312a(n) = Sum_{k=0..floor(n/3)} binomial(3*k,3*(n-3*k)).at n=26A391904