718187domain: NPropertiesPrimalityPrimeyesAppears in sequencesa(n) is the decimal expansion of 7nn7.at n=18A100897Primes of the concatenated form 7nn7.at n=1A102501a(n) is the least K such that K*6*M(n) - 1 and K*6*M(n) + 1 are twin primes, where M(n) = n-th Mersenne prime.at n=21A126612Prime numbersat n=57901