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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 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.)