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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 (list; graph; refs; listen; history; text; internal format)
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

Table of n, a(n) for n=1..26.

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

Cf. A007691, A054030, A058959.

Sequence in context: A101964 A294812 A217354 * A129305 A278507 A113631

Adjacent sequences:  A165698 A165699 A165700 * A165702 A165703 A165704

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

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Last modified August 3 05:55 EDT 2020. Contains 336197 sequences. (Running on oeis4.)