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%I #20 Sep 08 2022 08:46:14
%S 7,13,19,31,157,761,3469,6883,27677
%N Numbers k > 0 such that 10^(k+4) - 23 is prime.
%C Numbers k such that '9977' appended to k times the digit 9 is prime.
%C Up to a(8) the terms themselves are primes.
%F a(n) mod 2 = 1. - _Altug Alkan_, Dec 14 2015
%e 7 appears because 99999999977 ('9' concatenated 7 times and prepended to '9977') is prime.
%p A265629:=n->`if`(isprime((10^(n+4) - 23), n, NULL): seq(A265629(n), n=1..1000)
%t Select[ Range[10^3], PrimeQ[10^(# + 4) - 23] &]
%o (Magma) [n: n in [1..200] | IsPrime((10^(n+4) - 23)];
%o (PARI) is(n)=isprime(10^(n+4) - 23)
%Y Cf. A260903.
%K nonn,base,more
%O 1,1
%A _Mikk Heidemaa_, Dec 11 2015
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