Today I stumbled upon the term abundancy, as applied in a mathematical sense to the abundancy of a number and defined as the ratio:$$ \frac{\sigma(n)}{n} \text{ where } \sigma(n) \text{ is the divisor function}$$For \(n=1, 2, ...\), the first few values are:$$1, 3/2, 4/3, 7/4, 6/5, 2, 8/7, 15/8, 13/9, 9/5, 12/11, 7/3, 14/13, ...$$It's interesting to look at the approximate decimal value of this ratio as the numbers range from one up to a million. My analysis using SageMathCell revealed the following records for the maximum abundancy:
- 60 --> 14/5 \( \approx \) 2.80000000000000 between 1 and 100
- 840 --> 24/7 \( \approx \) 3.42857142857143 between 1 and 1000
- 5040 --> 403/105 \( \approx \) 3.83809523809524 between 1 and 10000
- 55440 --> 1612/385 \( \approx \) 4.18701298701299 between 1 and 100000
- 720720 --> 248/55 \( \approx \) 4.50909090909091 between 1 and 1000000
If this ratio turns out to be a positive integer, then \(n\) is said to be a multiperfect number. The first few are:$$1, 6, 28, 120, 496, 672, 8128, ...$$corresponding to the abundancies of: $$1, 2, 2, 3, 2, 3, 2, 4, 4, ... $$So a formal definition of a multiperfect number might be that a number \(n\) is \(k\)-multiperfect (also called a \(k\)-multiply perfect number or \(k\)-pluperfect number) if: $$ \sigma(n)=kn \text{ for some integer } k \geq 2$$The value of \(k\) is called the class. The special case \(k=2\) corresponds to perfect numbers \(P_2\). Source. Figure 1 shows the first few examples of such classes with the second column representing the associated OEIS reference:
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| Figure 1 |
Let's move on to hyperperfect numbers. A number \(n\) is called \(k\)-hyperperfect if$$n=1+k \, \sum_i d_i=1+k \,(\sigma(n)-n-1)$$where \( \sigma(n)\) is the divisor function and the summation is over the proper divisors with \(1<d_i<n\). Source. Figure 2 shows a table of the first few hyperperfect numbers:
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| Figure 2 |
As can be seen, the \(k\)-hyperperfect numbers reduce to the perfect numbers when \(k=1\) and the multiperfect numbers when \(k=2\). For other values of \(k\), the two sets of numbers differ.
Just to confuse matters further, Figure 3 shows a diagram with other types of numbers that are related to abundance. As can be seen, the terms perfect, colossally abundant, superior highly composite, superabundant, highly composite, primitive abundant, highly abundant, deficient and abundant are used.
Some of these I'm already familiar with and relevant posts include:
In following the source of the image that I used in Figure 3, I came across another class of numbers called superperfect numbers. A superperfect number is defined by this source as a positive integer \(n\) that satisfies:$$\sigma^2(n)=\sigma(\sigma(n))=2n$$The first few superperfect numbers are:$$2, 4, 16, 64, 4096, 65536, 262144, 1073741824, ...$$which is OEIS A019279. The comment is made that:
If \(n\) is an even superperfect number, then \(n\) must be a power of \(2\), \(2k\), such that \(2^{k+1} − 1\) is a Mersenne prime. It is not known whether there are any odd superperfect numbers. An odd superperfect number \(n\) would have to be a square number such that either \(n\) or \( \sigma(n) \) is divisible by at least three distinct primes. There are no odd superperfect numbers below \(7×10^{24}\).
Perfect and superperfect numbers are examples of the wider class of \(m\)-superperfect numbers, which satisfy:$$ \sigma^m(n)=2n \text{ with }m=1 \text{ and } 2$$For \(m \geq 3\) there are no even \(m\)-superperfect numbers. The \(m\)-superperfect numbers are in turn examples of \((m,k)\)-superperfect numbers which satisfy:$$ \sigma^m(n)=kn$$With this notation, perfect numbers are \((1,2)\)-perfect, multiperfect numbers are \((1,k)\)-perfect, superperfect numbers are \((2,2)\)-perfect and \(m\)-superperfect numbers are \((m,2)\)-perfect numbers. Examples are shown in Figure 4.
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| Figure 4 |
I must confess that I head is spinning with all this nomenclature so I'll leave off there.
