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 A288823 Numbers k such that (86*10^k - 221)/9 is prime. 0
 1, 4, 7, 13, 17, 31, 47, 64, 226, 251, 268, 310, 352, 394, 478, 599, 1529, 1679, 11590, 12922, 13151, 18808, 47188, 52450, 83038, 93217, 128086, 154853, 175774 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS For k>1, numbers such that the digit 9 followed by k-2 occurrences of the digit 5 followed by the digits 31 is prime (see Example section). a(39) > 2*10^5. LINKS Makoto Kamada, Factorization of near-repdigit-related numbers. Makoto Kamada, Search for 95w31. EXAMPLE 4 is in this sequence because (86*10^4 - 221)/9 = 95531 is prime. Initial terms and primes associated: a(1) = 1, 71; a(2) = 4, 95531; a(3) = 7, 95555531; a(4) = 13, 95555555555531; a(5) = 17, 955555555555555531; etc. MATHEMATICA Select[Range[1, 100000], PrimeQ[(86*10^# - 221)/9] &] CROSSREFS Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269. Sequence in context: A265704 A110267 A049698 * A310823 A074136 A310824 Adjacent sequences:  A288820 A288821 A288822 * A288824 A288825 A288826 KEYWORD nonn,more,hard AUTHOR Robert Price, Jun 17 2017 EXTENSIONS a(27)-a(29) from Robert Price, Aug 31 2019 STATUS approved

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Last modified June 30 09:10 EDT 2022. Contains 354920 sequences. (Running on oeis4.)