A306507 - OEIS

A306507

a(n) = gcd(n!^2+1, sigma(n!)), where sigma() denotes the sum of the divisors.

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

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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)).

MATHEMATICA