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%I #23 Nov 25 2024 12:15:54
%S 1,2,3,7,18,27,61,721,873,2492,8713
%N Numbers k such that 5^k + 2*k is prime.
%C a(12) > 20000. - _Jon E. Schoenfield_, Apr 19 2018
%C a(12) > 40000. - _Michael S. Branicky_, Apr 13 2023
%C a(12) > 100000. - _Michael S. Branicky_, Nov 25 2024
%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