7450171domain: NPropertiesPrimalityPrimeyesAppears in sequencesPrimes p of the form sigma(2q-1) where q is a prime.at n=15A247836Primes p such that there is prime q with sigma(q+2) = p.at n=21A247955Prime numbers with locations of right angle turns in the Ulam square spiral that are vertices of isosceles right triangles.at n=18A270269Prime numbersat n=505161