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 A293398 Numbers k such that (728*10^k - 71)/9 is prime. 0
 0, 2, 5, 6, 11, 14, 41, 47, 51, 84, 164, 228, 293, 1178, 1212, 1292, 2099, 2694, 4446, 7382, 13638, 39428, 40971 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS For k > 0, numbers such that the digits 80 followed by k-1 occurrences of the digit 8 followed by the digit 1 is prime (see Example section). a(24) > 2*10^5. LINKS Makoto Kamada, Factorization of near-repdigit-related numbers. Makoto Kamada, Search for 808w1. EXAMPLE 2 is in this sequence because (728*10^2 - 71)/9 = 8081 is prime. Initial terms and primes associated: a(1) = 0, 73; a(2) = 2, 8081; a(3) = 5, 8088881; a(4) = 6, 80888881; a(5) = 11, 8088888888881; etc. MATHEMATICA Select[Range[0, 100000], PrimeQ[(728*10^# - 71)/9] &] PROG (PARI) isok(k) = isprime((728*10^k - 71)/9); \\ Altug Alkan, Oct 08 2017 CROSSREFS Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269. Sequence in context: A135013 A239447 A336527 * A180323 A103035 A088273 Adjacent sequences:  A293395 A293396 A293397 * A293399 A293400 A293401 KEYWORD nonn,more,hard AUTHOR Robert Price, Oct 08 2017 STATUS approved

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Last modified April 21 17:36 EDT 2021. Contains 343156 sequences. (Running on oeis4.)