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%I M0589 #24 Feb 12 2021 01:28:02
%S 1,2,3,4,8,10,14,20,22,26,30,38,39,49,54,58,70,81,84,87,102,111,140,
%T 159,207,224,328,358,360,447,484,908,1083,1242,1461,1705,4624,5199,
%U 7106,8667,11157,13130,28052,30483,32166,34447,36986,86578,89940,120433,140743,147308,159276,165543
%N Numbers k such that 10*3^k - 1 is prime.
%C a(56) > 2*10^5. - _Robert Price_, Mar 16 2014
%C All terms are verified primes (i.e., not probable primes). - _Robert Price_, Mar 16 2014
%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="http://www.jstor.org/stable/2005886">Some prime numbers of the forms 2*3^n+1 and 2*3^n-1</a>, Math. Comp., 26 (1972), 995-998.
%t Do[ If[ PrimeQ[ 10*3^n - 1], Print[n] ], {n, 1, 12901} ]
%o (PARI) v=[ ]; for(n=1,1900, if(isprime(10*3^n-1),v=concat(v,n),)); v
%K hard,nonn
%O 1,2
%A _N. J. A. Sloane_
%E More terms from _Robert G. Wilson v_, Feb 05 2001
%E a(42)-a(55) from _Robert Price_, Mar 16 2014
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