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%I M1325 #24 Jun 28 2023 14:36:51
%S 2,5,6,14,21,26,141,278,281,306,345,1365,2573,2661,4766,5385
%N Numbers k such that (2^(2k+1) - 2^(k+1) + 1)/5 is prime.
%D J. Brillhart et al., Factorizations of b^n +- 1. Contemporary Mathematics, Vol. 22, Amer. Math. Soc., Providence, RI, 2nd edition, 1985; and later supplements.
%D N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
%H J. Brillhart et al., <a href="http://dx.doi.org/10.1090/conm/022">Factorizations of b^n +- 1</a>, Contemporary Mathematics, Vol. 22, Amer. Math. Soc., Providence, RI, 3rd edition, 2002.
%H Victor Meally, <a href="/A006556/a006556.pdf">Letter to N. J. A. Sloane</a>, no date.
%H S. S. Wagstaff, Jr., <a href="http://www.cerias.purdue.edu/homes/ssw/cun/index.html">The Cunningham Project</a>
%t For[ i=1, i<=10000, i++, If[ PrimeQ[ ( 2^(2n+1) - 2^(n+1) + 1)/5 ], Print[ n ] ] ]
%t Select[Range[5400],PrimeQ[(2^(2#+1)-2^(#+1)+1)/5]&] (* _Harvey P. Dale_, Jun 28 2023 *)
%o (PARI) is(n)=ispseudoprime((2^(2*n+1) - 2^(n+1) + 1)/5) \\ _Charles R Greathouse IV_, Jun 13 2017
%K nonn,hard,more
%O 1,1
%A _N. J. A. Sloane_
%E More terms from Douglas R. Burke (dburke(AT)nevada.edu)
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