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A275522 Numbers k such that (28*10^k + 773)/9 is prime.

%I A275522 #18 May 26 2024 14:41:03
%S A275522 0,2,5,6,11,20,32,59,81,98,128,437,758,989,998,1403,1548,1907,1914,
%T A275522 2219,5414,9047,13196,18518,28382
%N A275522 Numbers k such that (28*10^k + 773)/9 is prime.
%C A275522 For k > 1, numbers k such that the digit 3 followed by k-2 occurrences of the digit 1 followed by the digits 97 is prime (see Example section).
%C A275522 a(26) > 10^5.
%H A275522 Makoto Kamada, <a href="https://stdkmd.net/nrr">Factorization of near-repdigit-related numbers</a>.
%H A275522 Makoto Kamada, <a href="https://stdkmd.net/nrr/prime/prime_difficulty.txt">Search for 31w97</a>.
%e A275522 5 is in this sequence because (28*10^5 + 773)/9 = 311197 is prime.
%e A275522 Initial terms and associated primes:
%e A275522 a(1) = 0, 89;
%e A275522 a(2) = 2, 397;
%e A275522 a(3) = 5, 311197;
%e A275522 a(4) = 6, 3111197;
%e A275522 a(5) = 11, 311111111197, etc.
%t A275522 Select[Range[0, 100000], PrimeQ[(28*10^# + 773)/9] &]
%o A275522 (Magma) [n: n in [0..500] | IsPrime((28*10^n+773) div 9)]; // _Vincenzo Librandi_, Aug 01 2016
%o A275522 (PARI) is(n)=ispseudoprime((28*10^n+773)/9) \\ _Charles R Greathouse IV_, Jun 13 2017
%Y A275522 Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269.
%K A275522 nonn,more
%O A275522 1,2
%A A275522 _Robert Price_, Jul 31 2016