Year-end appeal: Please make a donation to the OEIS Foundation to support ongoing development and maintenance of the OEIS. We are now in our 61st year, we have over 378,000 sequences, and we’ve reached 11,000 citations (which often say “discovered thanks to the OEIS”).
%I #25 Nov 26 2024 09:52:08
%S 2,4,16,24,32,40,120,124,144,172,186,376,700,1232,1264,2894,5054,
%T 14826,16498,18348,33416,58082,59972,64290,103662
%N Numbers k such that (5*3^k - 7)/2 is prime.
%C a(21) > 20000. - _Jinyuan Wang_, Jan 28 2020
%t Do[ If[ PrimeQ[ (5*3^n - 7)/2 ], Print[n] ], {n, 0, 10000} ]
%t Select[Range[5100],PrimeQ[(5*3^#-7)/2]&] (* _Harvey P. Dale_, Jul 15 2018 *)
%o (PARI) is(n)=ispseudoprime((5*3^n-7)/2) \\ _Charles R Greathouse IV_, Jun 13 2017
%o (Magma) [k: k in [0..1000] | IsPrime((5*3^k - 7) div 2)]; // _Jinyuan Wang_, Jan 28 2020
%Y Cf. A059621 ((5*3^k+7)/2 is prime).
%K nonn,more
%O 1,1
%A _Robert G. Wilson v_, Feb 19 2001
%E a(18)-a(20) from _Jinyuan Wang_, Jan 28 2020
%E a(21) from _Michael S. Branicky_, Apr 29 2023
%E a(22)-a(24) from _Michael S. Branicky_, Oct 22 2024
%E a(25) from _Michael S. Branicky_, Nov 26 2024