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 A271821 Numbers n such that (5*10^n-143)/3 is prime. 0
 3, 4, 5, 6, 10, 23, 30, 33, 64, 189, 207, 213, 463, 547, 1225, 1795, 3726, 3947, 4989, 5226, 9825, 11489, 12666, 14512, 19588, 28795, 29903, 31889, 71357 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS For n>1, numbers such that the digit 1 followed by n-2 occurrences of the digit 6 followed by the digits 19 is prime (see Example section). a(31) > 2*10^5. LINKS Makoto Kamada, Factorization of near-repdigit-related numbers. Makoto Kamada, Search for 16w19. EXAMPLE 4 is in this sequence because (5*10^4-143)/3 = 16619 is prime. Initial terms and primes associated: a(1) = 3, 1619; a(2) = 4, 16619; a(3) = 5, 166619; a(4) = 6, 1666619; a(5) = 10, 16666666619, etc. MATHEMATICA Select[Range[0, 100000], PrimeQ[(5*10^#-143)/3] &] PROG (PARI) lista(nn) = for(n=1, nn, if(ispseudoprime((5*10^n-143)/3), print1(n, ", "))); \\ Altug Alkan, Apr 14 2016 CROSSREFS Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269. Sequence in context: A099561 A110300 A143713 * A261459 A215249 A089912 Adjacent sequences:  A271818 A271819 A271820 * A271822 A271823 A271824 KEYWORD nonn,more AUTHOR Robert Price, Apr 14 2016 STATUS approved

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Last modified December 10 23:14 EST 2019. Contains 329909 sequences. (Running on oeis4.)