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A272929 Numbers k such that (8*10^k - 77)/3 is prime.

%I A272929 #20 May 26 2024 14:29:11
%S A272929 2,4,5,6,15,18,43,45,55,60,105,128,180,207,271,479,869,1220,1478,1937,
%T A272929 4003,4213,5503,9562,11388,13120,34049,47178,156371,271039
%N A272929 Numbers k such that (8*10^k - 77)/3 is prime.
%C A272929 For k > 1, numbers k such that the digit 2 followed by k-2 occurrences of the digit 6 followed by the digits 41 is prime (see Example section).
%C A272929 a(31) > 3*10^5.
%H A272929 Makoto Kamada, <a href="https://stdkmd.net/nrr">Factorization of near-repdigit-related numbers</a>.
%H A272929 Makoto Kamada, <a href="https://stdkmd.net/nrr/prime/prime_difficulty.txt">Search for 26w41</a>.
%e A272929 4 is in this sequence because (8*10^4 - 77)/3 = 26641 is prime.
%e A272929 Initial terms and associated primes:
%e A272929 a(1) = 2, 241;
%e A272929 a(2) = 4, 26641;
%e A272929 a(3) = 5, 266641;
%e A272929 a(4) = 6, 2666641;
%e A272929 a(5) = 15, 2666666666666641, etc.
%t A272929 Select[Range[1, 100000], PrimeQ[(8*10^# - 77)/3] &]
%o A272929 (PARI) lista(nn) = {for(n=1, nn, if(ispseudoprime((8*10^n - 77)/3), print1(n, ", ")));} \\ _Altug Alkan_, May 11 2016
%Y A272929 Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269.
%K A272929 nonn,more
%O A272929 1,1
%A A272929 _Robert Price_, May 10 2016
%E A272929 a(29) from _Robert Price_, Jul 07 2018
%E A272929 a(30) from _Robert Price_, Jul 02 2023