2505890

Appears in sequences

• Numbers n such that sigma(n-1) and sigma(n) - 1 are both primes.at n=14A270415
• a(1) = 2, a(n) = k + 1, where k is the least number greater than a(n-1) such that rad(k) | a(n-1), where rad(n) = A007947(n).at n=29A365324