The OEIS is supported by the many generous donors to the OEIS Foundation.
%I #13 May 03 2024 07:45:35
%S 1,2,4,7,10,11,17,20,65,70,811,947,1099,1487,1540,6617,15067,18433,
%T 19063,31462,131270
%N Numbers k such that (37*10^k + 341)/9 is prime.
%C For k > 1, numbers k such that the digit 4 followed by k-2 occurrences of the digit 1 followed by the digits 49 is prime (see Example section).
%C a(22) > 2*10^5.
%H Makoto Kamada, <a href="https://stdkmd.net/nrr">Factorization of near-repdigit-related numbers</a>.
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/prime/prime_difficulty.txt">Search for 41w49</a>.
%e 2 is in this sequence because (37*10^2 + 341)/9 = 449 is prime.
%e Initial terms and associated primes:
%e a(1) = 1, 79;
%e a(2) = 2, 449;
%e a(3) = 4, 41149;
%e a(4) = 7, 41111149;
%e a(5) = 10, 41111111149; etc.
%t Select[Range[0, 100000], PrimeQ[(37*10^# + 341)/9] &]
%Y Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269.
%K nonn,more,hard
%O 1,2
%A _Robert Price_, Nov 28 2017
%E a(21) from _Robert Price_, Aug 14 2018