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
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
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