cp's OEIS Frontend

A102740 Numbers k such that 7*10^k - 11 is prime.

%I A102740 #23 May 31 2024 22:11:28
%S A102740 1,6,7,9,10,11,16,42,53,78,321,699,1858,3425,4899,5734,11081,11675,
%T A102740 12136,14056,16074,77969,158465
%N A102740 Numbers k such that 7*10^k - 11 is prime.
%C A102740 a(24) > 2*10^5.
%C A102740 Numbers corresponding to terms <= 699 are certified primes. - _Klaus Brockhaus_, Feb 15 2005
%C A102740 For k > 1, numbers k such that the digit 6 followed by k-2 occurrences of the digit 9 followed by the digits 89 is prime (see Example section).
%H A102740 Makoto Kamada, <a href="https://stdkmd.net/nrr">Factorization of near-repdigit-related numbers</a>.
%H A102740 Makoto Kamada, <a href="https://stdkmd.net/nrr/prime/prime_difficulty.txt">Search for 69w89</a>.
%e A102740 Initial terms and associated primes:
%e A102740 a(1) = 1, 59;
%e A102740 a(2) = 6, 6999989;
%e A102740 a(3) = 7, 69999989;
%e A102740 a(4) = 9, 6999999989;
%e A102740 a(5) = 10, 69999999989; etc.
%t A102740 Select[Range[1, 100000], PrimeQ[7*10^# - 11] &]
%o A102740 (PARI) is(n)=ispseudoprime(7*10^n - 11) \\ _Charles R Greathouse IV_, Jun 13 2017
%Y A102740 Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269.
%K A102740 more,nonn
%O A102740 1,2
%A A102740 Tom Mueller (muel4503(AT)uni-trier.de), Feb 08 2005
%E A102740 a(11)-a(13) from _Klaus Brockhaus_, Feb 15 2005
%E A102740 a(14)-a(22) from _Robert Price_, Oct 29 2017
%E A102740 a(23) from _Robert Price_, Jun 21 2019