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A063679 Numbers n such that (3^n-7)/2 is prime. 2
4, 12, 18, 26, 106, 164, 246, 956, 2554, 3350, 6496, 8706, 9008 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

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

LINKS

Table of n, a(n) for n=1..13.

EXAMPLE

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

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 *)

PROG

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

CROSSREFS

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

Sequence in context: A230628 A074285 A057311 * A075867 A071929 A008037

Adjacent sequences:  A063676 A063677 A063678 * A063680 A063681 A063682

KEYWORD

nonn

AUTHOR

Jud McCranie, Jul 28 2001

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

STATUS

approved

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Last modified February 23 00:03 EST 2018. Contains 299472 sequences. (Running on oeis4.)