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This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.

A271361 Numbers k such that 63*10^k + 1 is prime.

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%I A271361 #11 May 02 2024 22:54:59
%S A271361 1,2,12,21,27,30,33,44,46,76,78,83,84,92,582,750,787,3218,3290,5617,
%T A271361 6385,13960,22705,27636,36497,50349,51169,70381,70486,73096
%N A271361 Numbers k such that 63*10^k + 1 is prime.
%C A271361 Numbers k such that the digits 63 followed by k-1 occurrences of the digit 0 followed by the digit 1 is prime (see Example section).
%C A271361 a(31) > 10^5.
%H A271361 Makoto Kamada, <a href="https://stdkmd.net/nrr/prime/prime_difficulty.txt">Search for 630w1</a>.
%e A271361 2 is in this sequence because 63*10^2+1 = 6301 is prime.
%e A271361 Initial terms and associated primes:
%e A271361 a(1) = 1, 631;
%e A271361 a(2) = 2, 6301;
%e A271361 a(3) = 12, 63000000000001;
%e A271361 a(4) = 21, 63000000000000000000001;
%e A271361 a(5) = 27, 63000000000000000000000000001, etc.
%t A271361 Select[Range[0, 100000], PrimeQ[63*10^#+1] &]
%o A271361 (PARI) is(n)=ispseudoprime(63*10^n+1) \\ _Charles R Greathouse IV_, Jun 13 2017
%Y A271361 Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269.
%K A271361 nonn,more
%O A271361 1,2
%A A271361 _Robert Price_, Apr 05 2016