70111621domain: NAppears in sequencesNumbers n such that sigma(n) - n = {number of primes <= n}.at n=13A037172Numbers k such that us(k) = primepi(k), where us(k) is the sum of the aliquot unitary divisors of k (A034460), and primepi(k) is the number of primes <= k (A000720).at n=19A037176