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 A295399 Numbers k such that (8*10^k - 611)/9 is prime. 0
 2, 3, 4, 7, 9, 48, 52, 105, 153, 1783, 1999, 2787, 4276, 13693, 14067, 19143, 20431, 25933, 26206, 29062, 40462, 40791, 58738, 81709 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS For k > 1, numbers such that k-2 occurrences of the digit 8 followed by the digits 21 is prime (see Example section). a(25) > 2*10^5. LINKS Makoto Kamada, Factorization of near-repdigit-related numbers. Makoto Kamada, Search for 8w21 EXAMPLE 2 is in this sequence because (8*10^2 - 611)/9 = 21 is prime. Initial terms and primes associated: a(1) = 2, 21; a(2) = 3, 821; a(3) = 4, 8821; a(4) = 7, 8888821; a(5) = 9, 888888821; etc. MATHEMATICA Select[Range[2, 100000], PrimeQ[(8*10^# - 611)/9] &] PROG (PARI) isok(k) = isprime((8*10^k - 611)/9); \\ Michel Marcus, Nov 22 2017 CROSSREFS Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269. Sequence in context: A071114 A078696 A256772 * A274976 A269729 A098115 Adjacent sequences:  A295396 A295397 A295398 * A295400 A295401 A295402 KEYWORD nonn,more,hard AUTHOR Robert Price, Nov 21 2017 STATUS approved

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Last modified December 11 18:34 EST 2019. Contains 329925 sequences. (Running on oeis4.)