181191domain: NAppears in sequencesJuxtapose pairs of primes (starting at 1).at n=21A007794Concatenations of pairs of primes that differ by 10.at n=16A104719Semiprimes which are concatenation of two consecutive primes.at n=14A244057Let p = n-th prime == 3 mod 8; a(n) = sum of quadratic residues mod p that are > p/2.at n=44A282722