289561domain: NAppears in sequencesHypotenuse C of primitive Pythagorean triangles such that perimeters are Averages of twin prime pairs, q=p+1, a=q^2-p^2, c=q^2+p^2, b=2*p*q, s=a+b+c, s-+1 are primes.at n=25A155175Numbers k such that sigma(2*k-1) is a prime p.at n=23A247820