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A303090 Numbers k such that 5^k + 2*k is a prime. 1

%I

%S 1,2,3,7,18,27,61,721,873,2492,8713

%N Numbers k such that 5^k + 2*k is a prime.

%C a(12) > 20000. - _Jon E. Schoenfield_, Apr 19 2018

%e 1 is a term because 5^1 + 2*1 = 5 + 2 = 7, a prime.

%e 2 is a term because 5^2 + 2*2 = 25 + 4 = 29, a prime.

%e 3 is a term because 5^3 + 2*3 = 125 + 6 = 131, a prime.

%p a:=k->`if`(isprime(5^k+2*k),k,NULL): seq(a(k),k=1..5500);

%o (GAP) Filtered([1..700],k->IsPrime(5^k+2*k));

%o (PARI) isok(k) = isprime(5^k + 2*k); \\ _Altug Alkan_, Apr 19 2018

%Y Cf. A299642, A299643, A297663.

%K nonn,more

%O 1,2

%A _Muniru A Asiru_, Apr 18 2018

%E a(11) from _Jon E. Schoenfield_, Apr 18 2018

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Last modified January 28 13:14 EST 2020. Contains 331321 sequences. (Running on oeis4.)