I know that an odd perfect number cannot be divisible by $105$ or $825$. I wonder if that's also the case for $5313$.
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3Hint: Note that divisibility by $5313 = 3\cdot7\cdot11\cdot23$ implies divisibility by $3^47^411^4$. – Kortlek Apr 25 '14 at 15:46
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1@RonFord Interesting... – Anonymous - a group Apr 25 '14 at 15:48
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7What kind of group is Anonymous? :-) – Lucian Apr 25 '14 at 15:53
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4@Lucian It is a free group that is subject to no relations. – Anonymous - a group Apr 25 '14 at 16:04
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3@RonFord Also note that $$\left(\sum_{k=0}^4 \frac{1}{3^k}\right)\left(\sum_{k=0}^4 \frac{1}{7^k}\right)\left(\sum_{k=0}^4 \frac{1}{11^k}\right)\left(1 + \frac{1}{23}\right) > 2.$$ – Librecoin Apr 25 '14 at 16:17
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@Librecoin That's more of an answer than a note. You should post it as an answer! – Kortlek Apr 25 '14 at 16:21
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@RonFord Why is that an answer? – Anonymous - a group Apr 25 '14 at 16:24
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@Anonymous-agroup I think Librecoin can answer that best! – Kortlek Apr 25 '14 at 16:25
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1@Anonymous-agroup Because the abundancy index of a perfect number must be $2$. – Librecoin Apr 25 '14 at 16:26
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@Librecoin Thanks. – Anonymous - a group Apr 25 '14 at 16:27
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Librecoin and Ron Ford, can someone of you post that as an answer? – Anonymous - a group Apr 25 '14 at 16:28
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@Anonymous-agroup I will give Ron Ford the honor. It was his idea, after all. – Librecoin Apr 25 '14 at 16:29
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@RonFord You can make a community post if you don't want to take full credit. – Anonymous - a group Apr 25 '14 at 16:32
2 Answers
It is quite easy to prove that an odd perfect prime has a factorisation into powers of odd primes, where exactly one power is a prime of the form 4k+1, raised to a power 4n+1 (n can be 0), and all the other powers are even. This implies an odd perfect number which is a multiple of $5313 = 3 * 7 * 11 * 23$ must be a multiple of $3^27^211^223^2$. $3^2 7^2 11^2 23^2$ has an abundancy of 1.9037 < 2, so multiples of this number could a priori have an abundance of 2.
However, since $1 + 3 + 9 = 13$, if the number is not a multiple of $3^4$ then it must be a multiple of 13 (it doesn't have to be a multiple of $13^2$ because 13 = 4k+1). Since 1 + 7 + 49 = 57, if the number is not a multiple of $7^4$ then it must be a multiple of 3 (which it is) and of 19, and therefore a multiple of 19^2.
Each of the four possibilities $3^4 7^4 11^2 23^2$, $3^4 7^2 11^2 23^2 19^2$, $3^2 7^4 11^2 23^2 13$ and $3^2 7^2 11^2 23^2 19^2 13$ has an abundancy just above 2, so these numbers and any multiples of them cannot be perfect numbers.
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As $\textbf{Ron Ford}$ already pointed out in the comments, an odd perfect number can be divisible by $5313 = 3\cdot7\cdot11\cdot23$ only if it is divisible by $3^47^411^423$. To see this, use the same line of reasoning as in the linked question.
Continue by noting that
$$\left(\sum_{k=0}^4 \frac{1}{3^k}\right)\left(\sum_{k=0}^4 \frac{1}{7^k}\right)\left(\sum_{k=0}^4 \frac{1}{11^k}\right)\left(1 + \frac{1}{23}\right) > 2,$$
which implies that the abundancy index of the number must be larger than $2$. This contradicts the definition of a perfect number.
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2Voted it down because your proof is defective. There is no reason given why 3, 7 and 11 should be raised to the 4th power and not just squared. If you can give a reason then state it. The linked question doesn't give a justification either. – gnasher729 May 21 '14 at 07:23