Multiply Perfect Number

In mathematics, a multiply perfect number (also called multiperfect number or pluperfect number) is a generalization of a perfect number.

For a given natural number ''k'', a number ''n'' is called (or perfect) if the sum of all positive divisors of ''n'' (the divisor function, ''σ''(''n'')) is equal to ''kn''; a number is thus perfect if and only if it is . A number that is for a certain ''k'' is called a multiply perfect number. As of 2014, numbers are known for each value of ''k'' up to 11.

It is unknown whether there are any odd multiply perfect numbers other than 1. The first few multiply perfect numbers are:
:1, 6, 28, 120, 496, 672, 8128, 30240, 32760, 523776, 2178540, 23569920, 33550336, 45532800, 142990848, 459818240, ... .

Example

The sum of the divisors of 120 is
:1 + 2 + 3 + 4 + 5 + 6 + 8 + 10 + 12 + 15 + 20 + 24 + 30 + 40 + 60 + 120 = 360
which is 3 × 120. Therefore 120 is a number.

Smallest known ''k''-perfect numbers

The following table gives an overview of the smallest known numbers for ''k'' ≤ 11 :

Properties

It can be proven that:

* For a given prime number ''p'', if ''n'' is and ''p'' does not divide ''n'', then ''pn'' is . This implies that an integer ''n'' is a number divisible by 2 but not by 4, if and only if ''n''/2 is an odd perfect number, of which none are known.
* If 3''n'' is and 3 does not divide ''n'', then ''n'' is .

Odd multiply perfect numbers

It is unknown whether there are any odd multiply perfect numbers other than 1. However if an odd number ''n'' exists where ''k'' > 2, then it must satisfy the following conditions:
* The largest prime factor is ≥ 100129
* The second largest prime factor is ≥ 1009
* The third largest prime factor is ≥ 101

Tóth found several numbers that would be odd multiperfect, if one of their factors was a square (). An example is $8999757$, which would be an odd multiperfect number, if only one of its prime factors, $61$, was a square. This is closely related to the concept of Descartes numbers.

Bounds

In little-o notation, the number of multiply perfect numbers less than ''x'' is $o(x^\varepsilon)$ for all ε > 0.

The number of ''k''-perfect numbers ''n'' for ''n'' ≤ ''x'' is less than $cx^$, where ''c'' and ''c are constants independent of ''k''.

Under the assumption of the Riemann hypothesis, the following inequality is true for all numbers ''n'', where ''k'' > 3
:$\log\log n > k\cdot e^$
where $\gamma$ is Euler's gamma constant. This can be proven using Robin's theorem.

The number of divisors τ(''n'') of a number ''n'' satisfies the inequality
:$\tau(n) > e^.$

The number of distinct prime factors ω(''n'') of ''n'' satisfies
:$\omega(n) \ge k^2-1.$

If the distinct prime factors of ''n'' are $p_1, p_2, \ldots, p_r$, then:
:r \left(\sqrt - 1\right) < \sum_^ \frac < r \left(1 - \sqrt right), ~~ \textn\text
:r \left(\sqrt r- 1\right) < \sum_^ \frac < r \left(1 - \sqrt right), ~~ \textn\text

Specific values of ''k''

Perfect numbers

A number ''n'' with σ(''n'') = 2''n'' is perfect.

Triperfect numbers

A number ''n'' with σ(''n'') = 3''n'' is triperfect. There are only six known triperfect numbers and these are believed to comprise all such numbers:

: 120, 672, 523776, 459818240, 1476304896, 51001180160

If there exists an odd perfect number ''m'' (a famous open problem) then 2''m'' would be , since σ(2''m'') = σ(2)σ(''m'') = 3×2''m''. An odd triperfect number must be a square number exceeding 10⁷⁰ and have at least 12 distinct prime factors, the largest exceeding 10⁵.

Variations

Unitary multiply perfect numbers

A similar extension can be made for unitary perfect numbers. A positive integer ''n'' is called a unitary multi number if σ^*(''n'') = ''kn'' where σ^*(''n'') is the sum of its unitary divisors. (A divisor ''d'' of a number ''n'' is a unitary divisor if ''d'' and ''n/d'' share no common factors.).

A unitary multiply perfect number is simply a unitary multi number for some positive integer ''k''. Equivalently, unitary multiply perfect numbers are those ''n'' for which ''n'' divides σ^*(''n''). A unitary multi number is naturally called a unitary perfect number. In the case ''k'' > 2, no example of a unitary multi number is yet known. It is known that if such a number exists, it must be even and greater than 10¹⁰² and must have more than forty four odd prime factors. This problem is probably very difficult to settle. The concept of unitary divisor was originally due to R. Vaidyanathaswamy (1931) who called such a divisor as block factor. The present terminology is due to E. Cohen (1960).

The first few unitary multiply perfect numbers are:
:1, 6, 60, 90, 87360

Bi-unitary multiply perfect numbers

A positive integer ''n'' is called a bi-unitary multi number if σ^**(''n'') = ''kn'' where σ^**(''n'') is the sum of its bi-unitary divisors. This concept is due to Peter Hagis (1987). A bi-unitary multiply perfect number is simply a bi-unitary multi number for some positive integer ''k''. Equivalently, bi-unitary multiply perfect numbers are those ''n'' for which ''n'' divides σ^**(''n''). A bi-unitary multi number is naturally called a bi-unitary perfect number, and a bi-unitary multi number is called a bi-unitary triperfect number.

A divisor ''d'' of a positive integer ''n'' is called a bi-unitary divisor of ''n'' if the greatest common unitary divisor (gcud) of ''d'' and ''n''/''d'' equals 1. This concept is due to D. Surynarayana (1972). The sum of the (positive) bi-unitary divisors of ''n'' is denoted by σ^**(''n'').

Peter Hagis (1987) proved that there are no odd bi-unitary multiperfect numbers other than 1. Haukkanen and Sitaramaiah (2020) found all bi-unitary triperfect numbers of the form 2^''a''''u'' where 1 ≤ ''a'' ≤ 6 and ''u'' is odd, and partially the case where ''a'' = 7.

Further, they fixed completely the case ''a'' = 8.
Tomohiro Yamada (Determining all biunitary triperfect numbers of a certain form, arXiv:2406.19331 ath.NT 2024) proved that 2160 = 3³ 80 is the only biunitary triperfect number of the form 3^''a''''u'' where 3 ≤ ''a'' and ''u'' is not divisible by 3.

The first few bi-unitary multiply perfect numbers are:
:1, 6, 60, 90, 120, 672, 2160, 10080, 22848, 30240

References

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See also

* Hemiperfect number

External links

The Multiply Perfect Numbers page

The Prime Glossary: Multiply perfect numbers
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{{Classes of natural numbers

Arithmetic dynamics
Divisor function
Perfect numbers