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A306507 a(n) = gcd(n!^2+1, sigma(n!)), where sigma() denotes the sum of the divisors. 0
1, 1, 1, 1, 1, 13, 1, 17, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 61, 1, 1, 1, 1, 1, 1, 1, 1, 61, 1, 1, 1, 193, 1, 1, 1, 757, 61, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 109, 1, 1, 1, 181, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 113 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,6

COMMENTS

A sequence that produces primes.

A counterexample is found at n=7880, here the gcd is 380927609 = 15761*24169.

Interesting properties may be found in this sequence, for example many primes are 2n+1.

LINKS

Table of n, a(n) for n=1..85.

FORMULA

a(n) = gcd(A020549(n), A062569(n)).

MAPLE

with(numtheory): seq(gcd(n!^2+1, sigma(n!)), n=1..85); # Paolo P. Lava, Feb 21 2019

PROG

(PARI) a(n) = gcd(n!^2+1, sigma(n!)); \\ Michel Marcus, Feb 20 2019

CROSSREFS

Cf. A000203, A020549, A062569.

Sequence in context: A058018 A037283 A278634 * A094709 A236231 A040181

Adjacent sequences:  A306504 A306505 A306506 * A306508 A306509 A306510

KEYWORD

nonn

AUTHOR

Daoudi RĂ©doane, Feb 20 2019

EXTENSIONS

More terms from Michel Marcus, Feb 20 2019

STATUS

approved

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Last modified January 21 16:54 EST 2020. Contains 331114 sequences. (Running on oeis4.)