A063679 Numbers k such that (3^k - 7)/2 is prime.

4, 12, 18, 26, 106, 164, 246, 956, 2554, 3350, 6496, 8706, 9008, 15398, 15490, 20408, 39240, 41060, 41842, 58358, 60346, 82214, 134972, 194014, 344204, 587712, 778070

Offset: 1

Author

Jud McCranie, Jul 28 2001

Keywords

• nonn
• more

Comments

x = 3^k is a solution to sigma(x - 7) = sigma(x) - 7 when (3^k - 7)/2 is prime.

a(28) > 10^6

Examples

			(3^4 - 7)/2 = 37 is prime, so 4 is in the sequence.
		

Crossrefs

Cf. A000203, A063680, A015913-A015917, A054905, A116970.

Programs

Maple

with(numtheory):i := 0:x := 1:while i < 1000 do i := i+1:x := 3*x: if isprime((x-7)/2) then print(i):fi:od:

Mathematica

Do[ If[ PrimeQ[ (3^n - 7)/2 ], Print[n] ], {n, 2, 5500} ]
Select[Range[2, 10000], PrimeQ[((3^# - 7)/2)] &] (* Vincenzo Librandi, Sep 30 2012 *)

PARI

is(n)=ispseudoprime((3^n-7)/2) \\ Charles R Greathouse IV, May 22 2017

Extensions

More terms from Robert G. Wilson v, Aug 02 2001

0, 1 removed and a(11)-a(13) added from Vincenzo Librandi, Sep 30 2012

a(14)-a(17) from Seth A. Troisi, Oct 17 2022

a(17) corrected, a(18)-a(25) from Seth A. Troisi, Oct 29 2022

a(26)-a(27) from Seth A. Troisi, Nov 28 2022