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A172995 Numbers k such that (3^k + 7)/2 is prime. 2
1, 3, 7, 75, 191, 395, 891, 2935, 3855, 59887 (list; graph; refs; listen; history; text; internal format)
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

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Last modified February 7 02:40 EST 2023. Contains 360111 sequences. (Running on oeis4.)