

A172995


Numbers k such that (3^k + 7)/2 is prime.


2




OFFSET

1,2


COMMENTS

The primes are in A172996.
Naturally these numbers are odd since 3^(2*k) + 7 = (8+1)^k + 7 == 0 (mod 8).  Bruno Berselli, Oct 08 2012
For n <= 9, (3^a(n) + 7)/2 has been proved prime using PARI's ECPP; a(10) corresponds to a BPSW PRP. a(11) > 10^5.  Lucas A. Brown, Feb 10 2021


LINKS

Table of n, a(n) for n=1..10.
Lucas A. Brown, A172995.py


MATHEMATICA

Select[Range[5000], PrimeQ[(3^# + 7) / 2 ]&] (* Vincenzo Librandi, Oct 05 2012 *)


PROG

(PARI) is(n)=ispseudoprime((3^n+7)/2) \\ Charles R Greathouse IV, Jun 13 2017


CROSSREFS

Sequence in context: A075611 A191485 A119572 * A325476 A234536 A103737
Adjacent sequences: A172992 A172993 A172994 * A172996 A172997 A172998


KEYWORD

nonn,hard,more


AUTHOR

Vincenzo Librandi, Feb 07 2010


EXTENSIONS

a(7)(9) from Vincenzo Librandi, Oct 05 2012
a(10) from Lucas A. Brown, Feb 10 2021


STATUS

approved



