2162721domain: NAppears in sequencesIntersection of A137409 and A339870: Composite numbers k of the form 4u+1 having more than one prime factor of type 4u+3, and for which the odd part of phi(k) divides k-1.at n=17A339875Fermat pseudoprimes to base 2 that are palindromic in base 2.at n=18A346567