A063680 -id:A063680 - cp's OEIS frontend

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

Jud McCranie, Jul 28 2001

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

a(28) > 10^6

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

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

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:

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 *)

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

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

A063681 Primes p such that 2p+7 is a power of three.

Original entry on oeis.org

37, 265717, 193710241, 1270932914161, 187855106306818130162790081799568953899918191769361, 884821727139988111281341588371104523273965529248966451172919369920848760385637

Offset: 1

Jud McCranie, Jul 29 2001

nonn

If (3^i-7)/2 is prime then x=3^i-7 is a solution of sigma(x)+7 = sigma(x+7). See A063680 and indices resulting in primes are in A063679.

The next term (a(7)) has 118 digits. - Harvey P. Dale, May 10 2014

Cf. A063679, A063680.

Select[(#-7)/2&/@(3^Range[2,250]),PrimeQ] (* Harvey P. Dale, May 10 2014 *)

2*37+7 = 3^4, so 37 is in the sequence.

cp's OEIS Frontend

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

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