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 A290578 Numbers k such that (379*10^k - 1)/9 is prime. 0
 1, 2, 5, 8, 10, 11, 20, 56, 161, 172, 263, 290, 578, 800, 1166, 3382, 3848, 7036, 10487, 12101, 36211, 94337 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS For k>0, numbers such that the digits 42 followed by k occurrences of the digit 1 is prime (see Example section). a(23) > 10^5. LINKS Makoto Kamada, Factorization of near-repdigit-related numbers. Makoto Kamada, Search for 421w. EXAMPLE 2 is in this sequence because (379*10^2 - 1)/9 = 4211 is prime. Initial terms and primes associated: a(1) = 1, 421; a(2) = 2, 4211; a(3) = 5, 4211111; a(4) = 8; 4211111111; a(5) = 10, 421111111111; etc. MATHEMATICA Select[Range[0, 100000], PrimeQ[(379*10^# - 1)/9] &] PROG (PARI) isok(k) = isprime((379*10^k - 1)/9); \\ Michel Marcus, Aug 07 2017 CROSSREFS Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269. Sequence in context: A080228 A153052 A237525 * A294127 A166955 A286808 Adjacent sequences:  A290575 A290576 A290577 * A290579 A290580 A290581 KEYWORD nonn,more,hard AUTHOR Robert Price, Aug 06 2017 STATUS approved

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Last modified October 23 12:54 EDT 2019. Contains 328345 sequences. (Running on oeis4.)