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A063679
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Numbers n such that (3^n-7)/2 is prime.
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2
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4, 12, 18, 26, 106, 164, 246, 956, 2554, 3350, 6496, 8706, 9008
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OFFSET
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1,1
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COMMENTS
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x = 3^n is a solution to sigma(x - 7) = sigma(x) - 7 when (3^n - 7)/2 is prime.
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LINKS
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Table of n, a(n) for n=1..13.
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EXAMPLE
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(3^4-7)/2 = 37 is prime, so 4 is in the sequence.
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MAPLE
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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:
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MATHEMATICA
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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 *)
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PROG
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(PARI) is(n)=ispseudoprime((3^n-7)/2) \\ Charles R Greathouse IV, May 22 2017
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CROSSREFS
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Cf. A000203, A063680, A015913-A015917, A054905.
Sequence in context: A074285 A301252 A057311 * A075867 A071929 A008037
Adjacent sequences: A063676 A063677 A063678 * A063680 A063681 A063682
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KEYWORD
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nonn
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AUTHOR
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Jud McCranie, Jul 28 2001
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EXTENSIONS
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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
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STATUS
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approved
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