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 A271645 Numbers k such that (23*10^k + 91)/3 is prime. 0
 1, 2, 4, 15, 16, 19, 20, 26, 38, 47, 52, 75, 122, 191, 246, 257, 294, 305, 374, 592, 682, 729, 1092, 2053, 2997, 4065, 13936, 17214, 19059, 37433, 142105, 214633, 242909 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS For k>1, numbers such that the digit 7 followed by k-2 occurrences of the digit 6 followed by the digits 97 is prime (see Example section). a(34) > 3*10^5. LINKS Table of n, a(n) for n=1..33. Makoto Kamada, Factorization of near-repdigit-related numbers. Makoto Kamada, Search for 76w97. EXAMPLE 4 is in this sequence because (23*10^4 + 91)/3 = 76697 is prime. Initial terms and primes associated: a(1) = 1, 107; a(2) = 2, 797; a(3) = 4, 76697; a(4) = 15, 7666666666666697; a(5) = 16, 76666666666666697, etc. MATHEMATICA Select[Range[0, 100000], PrimeQ[(23*10^# + 91)/3] &] PROG (PARI) is(n)=ispseudoprime((23*10^n + 91)/3) \\ Charles R Greathouse IV, Jun 13 2017 CROSSREFS Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269. Sequence in context: A196239 A019543 A304568 * A265483 A357692 A154906 Adjacent sequences: A271642 A271643 A271644 * A271646 A271647 A271648 KEYWORD nonn,more AUTHOR Robert Price, Apr 11 2016 EXTENSIONS a(31) from Robert Price, Aug 11 2019 a(32)-a(33) from Robert Price, May 31 2023 STATUS approved

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Last modified February 29 23:21 EST 2024. Contains 370428 sequences. (Running on oeis4.)