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 A267420 Integers k such that 19*(10^k)+1 is prime. 1
 1, 2, 3, 11, 18, 25, 60, 71, 85, 168, 285, 627, 872, 1092, 1101, 1169, 1974, 2011, 2135, 2405, 10147, 14029, 16121, 17501, 46548 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS For k>0, numbers n such that the digits 19 followed by k-1 occurrences of the digit 0 followed by the digit 1 is prime (see Example section). a(26) > 10^5. LINKS Makoto Kamada, Factorization of near-repdigit-related numbers. Makoto Kamada, Search for 190w1. EXAMPLE 3 is in this sequence because 19*10^3 + 1 = 19001 is prime. Initial terms and associated primes: a(1) = 1, 191; a(2) = 2, 1901; a(3) = 3, 19001; a(4) = 11, 1900000000001; a(5) = 18, 19000000000000000001, etc. MAPLE select(k->isprime(19*(10^k)+1), [\$1..3000]); # Muniru A Asiru, Oct 07 2018 MATHEMATICA Select[Range[0, 100000], PrimeQ[19*10^# + 1] &] PROG (PARI) is(n) = ispseudoprime(19*(10^n) + 1); \\ Altug Alkan, Jan 15 2016 (PARI) lista(nn) = for(n=1, nn, if(isprime(19*(10^n) +1 ), print1(n, ", "))); \\ Altug Alkan, Jan 18 2016 CROSSREFS Sequence in context: A263937 A263938 A263932 * A322422 A024861 A025101 Adjacent sequences: A267417 A267418 A267419 * A267421 A267422 A267423 KEYWORD nonn,more AUTHOR Emre APARI, Jan 14 2016 EXTENSIONS a(13)-a(20) from Altug Alkan, Jan 18 2016 a(21)-a(25) from Robert Price, Oct 07 2018 STATUS approved

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Last modified November 30 08:36 EST 2022. Contains 358438 sequences. (Running on oeis4.)