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%I #18 Sep 08 2022 08:45:53
%S 5,41,401,4001,40000000000001
%N Primes of the form 4*10^n+1.
%H Vincenzo Librandi, <a href="/A177506/b177506.txt">Table of n, a(n) for n = 1..9</a>
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/4/40001.htm#prime">Prime numbers of the form 400...001</a>.
%t Select[Table[4 10^n + 1, {n, 0, 500}], PrimeQ] (* _Vincenzo Librandi_ Jan 02 2014 *)
%o (Magma) [a: n in [0..250] | IsPrime(a) where a is 4*10^n+1];
%Y Cf. A056806.
%K nonn
%O 1,1
%A _Vincenzo Librandi_, Dec 11 2010
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