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 A309774 Numbers k such that 5*10^k + 11 is prime. 0
 1, 3, 6, 35, 51, 84, 123, 1589, 1591, 4911, 6221, 48645, 78504, 107355 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS For k>1, numbers such that the digit 5 followed by k-2 occurrences of the digit 0 followed by the digits 11 is prime (see Example section). a(15) > 2*10^5. LINKS Makoto Kamada, Factorization of near-repdigit-related numbers. Makoto Kamada, Search for 50w11. EXAMPLE 6 is in this sequence because 5*10^4 + 11 = 5000011 is prime. Initial terms and primes associated: a(1) = 1, 61; a(2) = 3, 5011; a(3) = 6, 5000011; a(4) = 35, 500000000000000000000000000000000011; etc. MATHEMATICA Select[Range[0, 200000], PrimeQ[5*10^# + 11] &] CROSSREFS Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269. Sequence in context: A308557 A134748 A068904 * A244296 A143046 A246069 Adjacent sequences:  A309771 A309772 A309773 * A309775 A309776 A309777 KEYWORD nonn,more,hard AUTHOR Robert Price, Aug 16 2019 STATUS approved

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Last modified June 17 00:17 EDT 2021. Contains 345080 sequences. (Running on oeis4.)