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 A290330 Numbers k such that 8*10^k - 51 is prime. 0
 1, 3, 5, 28, 90, 209, 220, 226, 301, 352, 516, 1746, 2214, 2215, 2364, 2413, 3043, 3895, 5837, 8283, 16551, 20955, 46161, 67302, 82360, 94357, 100439 (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 9 followed by the digits 49 is prime (see Example section). a(28) > 2*10^5. LINKS Makoto Kamada, Factorization of near-repdigit-related numbers. Makoto Kamada, Search for 79w49. EXAMPLE 3 is in this sequence because 8*10^3 - 51 = 7949 is prime. Initial terms and primes associated: a(1) = 1, 29; a(2) = 3, 7949; a(3) = 5, 799949; a(4) = 28, 79999999999999999999999999949; etc. MATHEMATICA Select[Range[1, 100000], PrimeQ[8*10^# - 51] &] PROG (PARI) is(n) = ispseudoprime(8*10^n-51) \\ Felix FrÃ¶hlich, Aug 12 2019 CROSSREFS Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269. Sequence in context: A103526 A054388 A110255 * A300676 A016552 A321069 Adjacent sequences:  A290327 A290328 A290329 * A290331 A290332 A290333 KEYWORD nonn,more,hard AUTHOR Robert Price, Oct 06 2017 EXTENSIONS a(27) from Robert Price, Aug 12 2019 STATUS approved

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Last modified September 23 20:42 EDT 2021. Contains 347617 sequences. (Running on oeis4.)