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 A295082 Numbers k such that (17*10^k - 521)/9 is prime. 0
 2, 3, 5, 8, 9, 14, 20, 21, 29, 41, 113, 677, 3011, 3015, 3083, 3140, 4473, 4947, 138812 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS For k > 1, numbers such that the digit 1 followed by k-2 occurrences of the digit 8 followed by the digits 31 is prime (see Example section). a(20) > 2*10^5. LINKS Makoto Kamada, Factorization of near-repdigit-related numbers. Makoto Kamada, Search for 18w31 EXAMPLE 2 is in this sequence because (17*10^2 - 521)/9 = 131 is prime. Initial terms and associated primes: a(1) = 2, 131; a(2) = 3, 1831; a(3) = 5, 188831; a(4) = 8, 188888831; a(5) = 9, 1888888831; etc. MATHEMATICA Select[Range[2, 100000], PrimeQ[(17*10^# - 521)/9] &] PROG (PARI) isok(k) = isprime((17*10^k - 521)/9); \\ Michel Marcus, Nov 14 2017 CROSSREFS Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269. Sequence in context: A141399 A104737 A286495 * A168154 A278334 A120057 Adjacent sequences:  A295079 A295080 A295081 * A295083 A295084 A295085 KEYWORD nonn,more,hard AUTHOR Robert Price, Nov 13 2017 EXTENSIONS a(19) from Robert Price, Apr 24 2018 STATUS approved

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Last modified January 22 16:37 EST 2020. Contains 331152 sequences. (Running on oeis4.)