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%I #14 Jun 13 2017 10:21:01
%S 2,4,5,6,10,53,76,82,88,242,247,473,586,966,1015,1297,1825,2413,2599,
%T 2833,5850,5965,6052,27199,49704,79000
%N Numbers n such that 5^n-6 is prime.
%C Numbers corresponding to the a(n) for n>11 are probable prime.
%C If Q is a 4-perfect number and gcd(Q, 5*(5^a(n)-6))=1 then m=5^(a(n)-1)
%C (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.
%C a(27) > 10^5. - _Robert Price_, Feb 03 2014
%H F. Firoozbakht, M. F. Hasler, <a href="https://cs.uwaterloo.ca/journals/JIS/VOL13/Hasler/hasler2.html">Variations on Euclid's formula for Perfect Numbers</a>, JIS 13 (2010) #10.3.1
%H H. Lifchitz, R. Lifchitz: PRP Top Records <a href="http://www.primenumbers.net/prptop/searchform.php?form=5^n-6&action=Search">Search for 5^n-6</a>.
%t Do[If[PrimeQ[5^n-6],Print[n]],{n,8888}]
%o (PARI) is(n)=ispseudoprime(5^n-6) \\ _Charles R Greathouse IV_, Jun 13 2017
%Y Cf. A007691, A054030, A058959.
%K more,nonn
%O 1,1
%A M. F. Hasler and _Farideh Firoozbakht_, Oct 30 2009
%E a(24)-a(26) from _Robert Price_, Feb 03 2014
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