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%I #15 Jun 08 2024 05:44:06
%S 2,4,5,8,20,35,80,131,169,262,364,484,580,692,1687,1834,3476,3950,
%T 5963,6329,13339,18617,72703,103360
%N Numbers k such that (25*10^k - 241)/9 is prime.
%C For k > 1, numbers k such that the digit 2 followed by k-2 occurrences of the digit 7 followed by the digits 51 is prime (see Example section).
%C a(25) > 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 27w51</a>.
%e 4 is in this sequence because (25*10^4 - 241)/9 = 27751 is prime.
%e Initial terms and associated primes:
%e a(1) = 2, 251;
%e a(2) = 4, 27751;
%e a(3) = 5, 277751;
%e a(4) = 8, 277777751;
%e a(5) = 20, 277777777777777777751; etc.
%t Select[Range[1, 100000], PrimeQ[(25*10^# - 241)/9] &]
%o (PARI) isok(k) = ispseudoprime((25*10^k-241)/9); \\ _Altug Alkan_, May 07 2018
%Y Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269.
%K nonn,more,hard
%O 1,1
%A _Robert Price_, Jan 25 2017
%E a(24) from _Robert Price_, May 07 2018