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Numbers k such that prime(k) * 2^k + 1 is prime.
2

%I #9 Jul 14 2021 01:56:08

%S 1,2,3,4,5,9,12,35,78,122,276,495,715,849,927,1128,1263,1418,23201,

%T 30316,34894,51844,200821,235383,265970

%N Numbers k such that prime(k) * 2^k + 1 is prime.

%C The PFGW program has been used to certify all the terms up to a(25), using a deterministic test which exploits the factorization of a(n)-1.

%C a(26) > 325000.

%t Select[Range[1000], PrimeQ[Prime[#]* 2^# + 1] &]

%o (PARI) is(n,p=prime(n))=ispseudoprime(p<<n+1) \\ _Charles R Greathouse IV_, Jun 06 2017

%Y Cf. A239741.

%K nonn,more

%O 1,2

%A _Giovanni Resta_, Mar 26 2014