cp's OEIS Frontend

A267865 Numbers k such that 49*10^k + 1 is prime.

%I A267865 #16 May 03 2024 07:04:53
%S A267865 1,4,5,7,9,10,18,19,34,52,71,180,238,331,370,576,925,994,1075,1841,
%T A267865 2460,2857,6007,6193,10836,18732,58708,72861
%N A267865 Numbers k such that 49*10^k + 1 is prime.
%C A267865 Numbers k such that the digits 49 followed by k-1 occurrences of the digit 0 followed by the digit 1 is prime (see Example section).
%C A267865 a(29) > 10^5.
%H A267865 Makoto Kamada, <a href="https://stdkmd.net/nrr">Factorization of near-repdigit-related numbers</a>.
%H A267865 Makoto Kamada, <a href="https://stdkmd.net/nrr/prime/prime_difficulty.txt">Search for 490w1</a>.
%e A267865 4 is in this sequence because 49*10^4 + 1 = 490001 is prime.
%e A267865 Initial terms and associated primes:
%e A267865 a(1) = 1, 491;
%e A267865 a(2) = 4, 490001;
%e A267865 a(3) = 5, 4900001;
%e A267865 a(4) = 7, 490000001;
%e A267865 a(5) = 9, 49000000001, etc.
%t A267865 Select[Range[0, 100000], PrimeQ[49*10^#+1] &]
%o A267865 (PARI) is(n)=ispseudoprime(49*10^n+1) \\ _Charles R Greathouse IV_, Jun 13 2017
%Y A267865 Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269.
%K A267865 nonn,more
%O A267865 1,2
%A A267865 _Robert Price_, Apr 07 2016