cp's OEIS Frontend

A275978 Numbers k such that (101*10^k + 1)/3 is prime.

%I A275978 #18 May 26 2024 14:41:13
%S A275978 1,4,6,12,34,54,60,61,73,148,349,552,649,967,1044,2521,4501,5721,6133,
%T A275978 9052,9880,16126,16215,19146,61770
%N A275978 Numbers k such that (101*10^k + 1)/3 is prime.
%C A275978 Numbers k such that the digits 33 followed by k-1 occurrences of the digit 6 followed by the digit 7 is prime (see Example section).
%C A275978 a(26) > 10^5.
%H A275978 Makoto Kamada, <a href="https://stdkmd.net/nrr">Factorization of near-repdigit-related numbers</a>.
%H A275978 Makoto Kamada, <a href="https://stdkmd.net/nrr/prime/prime_difficulty.txt">Search for 336w7</a>.
%e A275978 4 is in this sequence because (101*10^4 + 1)/3 = 336667 is prime.
%e A275978 Initial terms and associated primes:
%e A275978 a(1) = 1, 337;
%e A275978 a(2) = 4, 336667;
%e A275978 a(3) = 6, 33666667;
%e A275978 a(4) = 12, 33666666666667;
%e A275978 a(5) = 34, 336666666666666666666666666666666667, etc.
%t A275978 Select[Range[0, 100000], PrimeQ[(101*10^# + 1)/3] &]
%o A275978 (PARI) isok(n) = isprime((101*10^n + 1)/3); \\ _Michel Marcus_, Aug 16 2016
%Y A275978 Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269.
%K A275978 nonn,more
%O A275978 1,2
%A A275978 _Robert Price_, Aug 15 2016