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 A295631 Numbers k such that (38*10^k + 187)/9 is prime. 0
 2, 3, 5, 6, 9, 14, 17, 20, 54, 56, 165, 902, 1023, 6483, 14174, 18411, 20025, 27411, 49583, 59589, 66896, 97329 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS For k > 1, numbers such that the digit 4 followed by k-2 occurrences of the digit 2 followed by the digits 43 is prime (see Example section). a(23) > 2*10^5. LINKS Makoto Kamada, Factorization of near-repdigit-related numbers. Makoto Kamada, Search for 42w43 EXAMPLE 2 is in this sequence because (38*10^2 + 187)/9 = 443 is prime. Initial terms and primes associated: a(1) = 2, 443; a(2) = 3, 4243; a(3) = 5, 422243; a(4) = 6, 4222243; a(5) = 9, 4222222243; etc. MATHEMATICA Select[Range[0, 100000], PrimeQ[38*10^# + 187)/9] &] Select[Range[2, 100000], PrimeQ[100*FromDigits[PadRight[{4}, #-1, 2]]+43]&] (* Harvey P. Dale, Jan 16 2018 *) PROG (PARI) is(k) = ispseudoprime((38*10^k + 187)/9) \\ Iain Fox, Nov 24 2017 CROSSREFS Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269. Sequence in context: A175095 A073216 A181902 * A278707 A294631 A059454 Adjacent sequences:  A295628 A295629 A295630 * A295632 A295633 A295634 KEYWORD nonn,more,hard AUTHOR Robert Price, Nov 24 2017 STATUS approved

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Last modified September 20 19:48 EDT 2020. Contains 337265 sequences. (Running on oeis4.)