cp's OEIS Frontend

A275067 Numbers k such that 7*10^k + 39 is prime.

%I A275067 #19 May 26 2024 15:03:51
%S A275067 1,2,3,4,12,19,26,32,84,164,199,251,306,510,641,1028,1147,1802,1948,
%T A275067 2058,2243,2257,4282,7900,7941,10179,10723,13570,20565,29132,34947,
%U A275067 63493,87319,107870,183511,183596,209161,227178,273983,287854
%N A275067 Numbers k such that 7*10^k + 39 is prime.
%C A275067 For k > 1, numbers k such that the digit 7 followed by k-2 occurrences of the digit 0 followed by the digits 39 is prime (see Example section).
%C A275067 a(41) > 3*10^5.
%H A275067 Makoto Kamada, <a href="https://stdkmd.net/nrr">Factorization of near-repdigit-related numbers</a>.
%H A275067 Makoto Kamada, <a href="https://stdkmd.net/nrr/prime/prime_difficulty.txt">Search for 70w39</a>.
%e A275067 3 is in this sequence because 7*10^3 + 39 = 7039 is prime.
%e A275067 Initial terms and associated primes:
%e A275067 a(1) = 1, 109;
%e A275067 a(2) = 2, 739;
%e A275067 a(3) = 3, 7039;
%e A275067 a(4) = 4, 70000039;
%e A275067 a(5) = 12, 7000000000039, etc.
%t A275067 Select[Range[0, 100000], PrimeQ[7*10^# + 39] &]
%o A275067 (PARI) is(n)=ispseudoprime(7*10^n+39) \\ _Charles R Greathouse IV_, Jun 13 2017
%Y A275067 Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269.
%K A275067 nonn,more
%O A275067 1,2
%A A275067 _Robert Price_, Jul 15 2016
%E A275067 a(34)-a(40) from _Robert Price_, Jul 02 2023