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 A291178 Numbers k such that (13*10^k - 37)/3 is prime. 0
 1, 2, 4, 7, 8, 26, 64, 116, 123, 157, 178, 288, 328, 1730, 2712, 3244, 3865, 7766, 8792, 9512, 14917, 33912, 39058, 57997, 120306, 150675, 171306, 173467, 175965 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS For k>1, numbers such that the digit 4 followed by k-2 occurrences of the digit 3 followed by the digits 21 is prime (see Example section). a(30) > 2*10^5. LINKS Makoto Kamada, Factorization of near-repdigit-related numbers. Makoto Kamada, Search for 43w21. EXAMPLE 4 is in this sequence because (13*10^4 - 37)/3 = 43321 is prime. Initial terms and primes associated: a(1) = 1, 31; a(2) = 2, 421; a(3) = 4, 43321; a(4) = 7, 43333321; a(5) = 8, 433333321; etc. MATHEMATICA Select[Range[2, 100000], PrimeQ[(13*10^# - 37)/3] &] PROG (PARI) isok(n) = isprime((13*10^n - 37)/3); \\ Altug Alkan, Aug 21 2017 CROSSREFS Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269. Sequence in context: A122979 A122980 A012985 * A023145 A094446 A071790 Adjacent sequences:  A291175 A291176 A291177 * A291179 A291180 A291181 KEYWORD nonn,more,hard AUTHOR Robert Price, Aug 19 2017 EXTENSIONS a(25)-a(29) from Robert Price, Jan 01 2019 STATUS approved

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Last modified May 10 05:05 EDT 2021. Contains 343748 sequences. (Running on oeis4.)