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A265629 Numbers k > 0 such that 10^(k+4) - 23 is prime. 2

%I

%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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Last modified May 28 05:47 EDT 2022. Contains 354112 sequences. (Running on oeis4.)