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 A063679 Numbers k such that (3^k - 7)/2 is prime. 3
 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 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 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 LINKS 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, A116970. Sequence in context: A074285 A301252 A057311 * A340597 A325234 A075867 Adjacent sequences: A063676 A063677 A063678 * A063680 A063681 A063682 KEYWORD nonn,more 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 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 STATUS approved

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Last modified January 27 21:49 EST 2023. Contains 359849 sequences. (Running on oeis4.)