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 A278695 Numbers k such that (38*10^k - 119) / 9 is prime. 0
 1, 2, 4, 5, 8, 14, 16, 17, 20, 22, 38, 134, 136, 152, 829, 1087, 2110, 3524, 4412, 7079, 10538, 17672, 21583, 24163, 35126, 72050 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS For k>1, numbers such that the digit 4 followed by k-2 occurrences of the digit 2 followed by the digits 09 is prime (see Example section). a(27) > 2*10^5. LINKS Makoto Kamada, Factorization of near-repdigit-related numbers. Makoto Kamada, Search for 42w09. EXAMPLE 4 is in this sequence because (38*10^4 - 119) / 9 = 42209 is prime. Initial terms and primes associated: a(1) = 1, 29; a(2) = 2, 409; a(3) = 4, 42209; a(4) = 5, 422209; a(5) = 8, 422222209; etc. MATHEMATICA Select[Range[0, 100000], PrimeQ[(38*10^# - 119) / 9] &] PROG (PARI) isok(n) = isprime((38*10^n - 119) / 9); \\ Michel Marcus, Nov 27 2016 CROSSREFS Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269. Sequence in context: A288668 A264800 A293189 * A105134 A226791 A184775 Adjacent sequences:  A278692 A278693 A278694 * A278696 A278697 A278698 KEYWORD nonn,more,hard,changed AUTHOR Robert Price, Nov 26 2016 STATUS approved

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Last modified January 19 16:18 EST 2019. Contains 319307 sequences. (Running on oeis4.)