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A165701
Numbers n such that 5^n-6 is prime.
3
2, 4, 5, 6, 10, 53, 76, 82, 88, 242, 247, 473, 586, 966, 1015, 1297, 1825, 2413, 2599, 2833, 5850, 5965, 6052, 27199, 49704, 79000
OFFSET
1,1
COMMENTS
Numbers corresponding to the a(n) for n>11 are probable prime.
If Q is a 4-perfect number and gcd(Q, 5*(5^a(n)-6))=1 then m=5^(a(n)-1)
(5^a(n)-6)*Q is a solution of the equation sigma(x)=5(x+Q)(see comment lines of the sequence A058959). 142990848 is the smallest 4-perfect number m such that 5 doesn't divide m.
a(27) > 10^5. - Robert Price, Feb 03 2014
LINKS
F. Firoozbakht, M. F. Hasler, Variations on Euclid's formula for Perfect Numbers, JIS 13 (2010) #10.3.1
H. Lifchitz, R. Lifchitz: PRP Top Records Search for 5^n-6.
MATHEMATICA
Do[If[PrimeQ[5^n-6], Print[n]], {n, 8888}]
PROG
(PARI) is(n)=ispseudoprime(5^n-6) \\ Charles R Greathouse IV, Jun 13 2017
CROSSREFS
KEYWORD
more,nonn
AUTHOR
M. F. Hasler and Farideh Firoozbakht, Oct 30 2009
EXTENSIONS
a(24)-a(26) from Robert Price, Feb 03 2014
STATUS
approved