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A005540 Numbers k such that 4*3^k - 1 is prime.
(Formerly M2422)

%I M2422

%S 0,1,3,5,7,15,45,95,235,463,733,1437,1583,1677,1803,4163,4765,9219,

%T 9959,25477,26059,41539,54195,65057,74977,116589,192289,311835,350767,

%U 353635,416337,423253

%N Numbers k such that 4*3^k - 1 is prime.

%D N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).

%H H. C. Williams and C. R. Zarnke, <a href="https://doi.org/10.1090/S0025-5718-1972-0314747-X">Some prime numbers of the forms 2*3^n+1 and 2*3^n-1</a>, Math. Comp., 26 (1972), 995-998.

%o (PARI) is(n)=ispseudoprime(4*3^n-1) \\ _Charles R Greathouse IV_, Feb 20 2017

%K nonn,hard

%O 1,3

%A _N. J. A. Sloane_

%E More terms from _Robert G. Wilson v_ and from _Jud McCranie_

%E More terms from _Borys Jaworski_, Feb 13 2010

%E 0 added by _Vincenzo Librandi_, Sep 26 2012

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Last modified February 17 12:14 EST 2020. Contains 331996 sequences. (Running on oeis4.)