cp's OEIS Frontend

A271340 Numbers k such that (14*10^k + 73)/3 is prime.

%I A271340 #21 Jun 06 2024 23:24:12
%S A271340 0,1,2,3,4,6,7,10,15,19,32,54,68,114,148,227,238,286,405,789,857,1310,
%T A271340 2314,3613,4103,4215,5135,6094,8023,8718,16899,34215,41989,81585,
%U A271340 85010,143097,165282,199447
%N A271340 Numbers k such that (14*10^k + 73)/3 is prime.
%C A271340 For k > 1, numbers k such that the digit 4 followed by k-2 occurrences of the digit 6 followed by the digits 91 is prime (see Example section).
%C A271340 a(39) > 2*10^5.
%H A271340 Makoto Kamada, <a href="https://stdkmd.net/nrr/prime/prime_difficulty.txt">Search for 46w91</a>.
%e A271340 3 is in this sequence because (14*10^3 + 73)/3 = 4691 is prime.
%e A271340 Initial terms and associated primes:
%e A271340 a(1) = 0, 29;
%e A271340 a(2) = 1, 71;
%e A271340 a(3) = 2, 491;
%e A271340 a(4) = 3, 4691;
%e A271340 a(5) = 4, 46691;
%e A271340 a(6) = 6, 4666691, etc.
%t A271340 Select[Range[0, 100000], PrimeQ[(14*10^# + 73)/3] &]
%o A271340 (PARI) is(n)=ispseudoprime((14*10^n + 73)/3) \\ _Charles R Greathouse IV_, Jun 13 2017
%Y A271340 Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271109, A271269.
%K A271340 nonn,more
%O A271340 1,3
%A A271340 _Robert Price_, Apr 05 2016
%E A271340 a(36)-a(38) from _Robert Price_, Dec 26 2018