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 A286936 Numbers k such that (71*10^k - 287)/9 is prime. 0
 1, 2, 4, 8, 26, 28, 43, 70, 92, 128, 331, 364, 478, 532, 689, 778, 895, 1210, 5081, 7855, 17852, 20864, 42598, 56858, 120703, 173854 (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 8 followed by the digits 57 is prime (see Example section). a(27) > 2*10^5. LINKS Makoto Kamada, Factorization of near-repdigit-related numbers. Makoto Kamada, Search for 78w57. EXAMPLE 4 is in this sequence because (71*10^4 - 287)/9 = 78857 is prime. Initial terms and primes associated: a(1) = 1, 47; a(2) = 2, 757; a(3) = 4, 78857; a(4) = 8, 788888857; a(5) = 26, 788888888888888888888888857; etc. MATHEMATICA Select[Range[1, 100000], PrimeQ[(71*10^# - 287)/9] &] CROSSREFS Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269. Sequence in context: A191700 A000643 A261489 * A323948 A196265 A112285 Adjacent sequences:  A286933 A286934 A286935 * A286937 A286938 A286939 KEYWORD nonn,more,hard AUTHOR Robert Price, May 16 2017 EXTENSIONS a(25)-a(26) from Robert Price, Sep 06 2019 STATUS approved

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Last modified March 30 18:21 EDT 2020. Contains 333127 sequences. (Running on oeis4.)