21362755051domain: NAppears in sequencesTriangular numbers that are one-third of another triangular number: T(m) such that 3*T(m) = T(k) for some k.at n=10A076139Numbers that are both triangular and pentagonal of the second kind.at n=5A108281Positive hexagonal numbers (A000384) that are other hexagonal numbers divided by 3.at n=2A259210