cp's OEIS Frontend

A276047 Numbers k such that 4*10^k + 21 is prime.

%I A276047 #20 Jun 03 2024 23:31:29
%S A276047 1,2,3,7,15,22,30,35,44,73,89,91,224,533,821,1037,1338,1458,1777,2046,
%T A276047 2257,2877,3047,3407,13398,42766,55906,61625,66653,123113,229836,
%U A276047 238163
%N A276047 Numbers k such that 4*10^k + 21 is prime.
%C A276047 For k > 1, numbers k such that the digit 4 followed by k-2 occurrences of the digit 0 followed by the digits 21 is prime (see Example section).
%C A276047 a(33) > 3*10^5.
%H A276047 Makoto Kamada, <a href="https://stdkmd.net/nrr">Factorization of near-repdigit-related numbers</a>.
%H A276047 Makoto Kamada, <a href="https://stdkmd.net/nrr/prime/prime_difficulty.txt">Search for 40w21</a>.
%e A276047 3 is in this sequence because 4*10^3 + 21 = 4021 is prime.
%e A276047 Initial terms and associated primes:
%e A276047 a(1) = 1, 61;
%e A276047 a(2) = 2, 421;
%e A276047 a(3) = 3, 4021;
%e A276047 a(4) = 7, 40000021;
%e A276047 a(5) = 15, 4000000000000021, etc.
%t A276047 Select[Range[0, 100000], PrimeQ[4*10^# + 21] &]
%o A276047 (PARI) is(n)=ispseudoprime(4*10^n + 21) \\ _Charles R Greathouse IV_, Jun 13 2017
%Y A276047 Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269.
%K A276047 nonn,more
%O A276047 1,2
%A A276047 _Robert Price_, Aug 17 2016
%E A276047 a(30) from _Robert Price_, Dec 09 2018
%E A276047 a(31) - a(32) from _Robert Price_, Jun 01 2023