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Primes of the form (5^k-1)/4.
7

%I #15 May 25 2019 01:54:55

%S 31,19531,12207031,305175781,177635683940025046467781066894531,

%T 14693679385278593849609206715278070972733319459651094018859396328480215743184089660644531,

%U 35032461608120426773093239582247903282006548546912894293926707097244777067146515037165954709053039550781

%N Primes of the form (5^k-1)/4.

%C Corresponding exponents k are listed in A004061. - _Alexander Adamchuk_, Jan 23 2007

%F a(n) = (5^A004061(n) - 1)/4 = A003463[ A004061(n) ]. - _Alexander Adamchuk_, Jan 23 2007

%F A003464 INTERSECT A000040.

%t Do[f=(5^n-1)/4;If[PrimeQ[f],Print[{n,f}]],{n,1,1000}] (* _Alexander Adamchuk_, Jan 23 2007 *)

%t Select[(5^Range[300]-1)/4,PrimeQ] (* _Harvey P. Dale_, Dec 11 2016 *)

%Y Cf. A000668, A076481.

%Y Cf. A003463, A004061, A074479.

%K nonn

%O 1,1

%A _Labos Elemer_, Jul 23 2003

%E More terms from _Alexander Adamchuk_, Jan 23 2007