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Numbers k such that 7*k - 4 is prime.
7

%I #21 Sep 08 2022 08:44:48

%S 1,3,5,9,11,15,23,29,33,35,39,41,45,51,53,59,69,75,81,83,89,93,95,111,

%T 113,119,123,135,141,143,149,159,161,165,171,179,183,185,189,195,209,

%U 213,221,225,231,233,239,243,251,261,269,273,279,299,305,321,335,341

%N Numbers k such that 7*k - 4 is prime.

%H Vincenzo Librandi, <a href="/A024903/b024903.txt">Table of n, a(n) for n = 1..1000</a>

%F a(n) = A033868(n-1)-1 = A089033(n-1)+1 = A090614(n-1)*2+1.

%t Select[Range[350], PrimeQ[7 # - 4] &] (* _Vincenzo Librandi_, Sep 26 2012 *)

%o (Magma) [n: n in [1..400] | IsPrime(7*n-4)]; // _Vincenzo Librandi_, Nov 20 2010

%o (PARI) is(n)=isprime(7*n-4) \\ _Charles R Greathouse IV_, Feb 17 2017

%Y Cf. A045437 (associated primes), A090613 (gives prime index).

%Y Cf. A033868, A089033, A090614.

%K nonn,easy

%O 1,2

%A _Clark Kimberling_