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 A289942 Numbers k such that (85*10^k - 1)/3 is prime. 0
 1, 2, 5, 9, 64, 69, 118, 377, 409, 616, 959, 1093, 1726, 3034, 5929, 8140, 11197, 15293, 15381, 27811, 58091, 63757, 72025, 83921 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Numbers such that the digits 28 followed by k occurrences of the digit 3 is prime (see Example section). a(25) > 10^5. LINKS Makoto Kamada, Factorization of near-repdigit-related numbers. Makoto Kamada, Search for 283w. EXAMPLE 2 is in this sequence because (85*10^2 - 1)/3 = 2833 is prime. Initial terms and primes associated: a(1) = 1, 283; a(2) = 2, 2833; a(3) = 5, 2833333; a(4) = 9, 28333333333; a(5) = 64, 283333333333333333333333333333333333333333333333333333333333333333; etc. MATHEMATICA Select[Range[0, 100000], PrimeQ[(85*10^# - 1)/3] &] PROG (PARI) isok(k) = isprime((85*10^k - 1)/3); \\ Michel Marcus, Jul 16 2017 CROSSREFS Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269. Sequence in context: A249583 A109469 A185160 * A239899 A188948 A083460 Adjacent sequences:  A289939 A289940 A289941 * A289943 A289944 A289945 KEYWORD nonn,more,hard AUTHOR Robert Price, Jul 15 2017 STATUS approved

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Last modified January 26 05:10 EST 2020. Contains 331273 sequences. (Running on oeis4.)