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Abundant numbers are numbers whose factors add up to a higher number than the number itself. Confused? ...Good. Then sit back and listen.

Think of the number 12, and think of its factors. They are 1, 2, 3, 4 and 6. They add up to 16. 16 is higher than 12, so 12 is an abundant number.
Think of the number 24. 24's factors are 1, 2, 3, 4, 6, 8 and 12. They add up to 36. 36 is higher than 24, so 24 is abundant.

Got the picture? ...Good.

Now there are a couple of little tricks about abundant numbers that might impress your maths teacher even more. Ready? ...OK.

If a number's factors add up to the number itself, it is called a perfect number. Perfect numbers are very very rare indeed. There are only 3 below 500: 6, 28 and 496.

6's factors are 1, 2 and 3. Those three add up to make 6, so 6 is perfect, as is 28, with its factors being 1, 2, 4, 7 and 14. Don't ask me about 496 - work it out for yourself.

A weird number is an abundant number with a special little twist. If its factors add up to a number higher than the number it's abundant. If no combination of its factors add up to the number itself, it is also weird. Take a look at 12, 24 and 70:
You can get 12 from 6, 4 and 2, which are all factors of 12, so 12 is not weird.
You can get 24 from 12, 8 and 4, which are all factors of 24, so 24 is not weird. (But the TV show is.)
Try and get 70 out of 1, 2, 5, 7, 10, 14 and 35. I think you'll find it quite difficult. Therefore, 70 is weird.

Have a look for more abundant numbers - they're everywhere.

The search for perfect numbers loses its cachet after a while, when you realize that all the known perfect numbers are even and were found using a formula, and any outside the formula are exceedingly unlikely. We can interest ourselves in aliquot parts for a little while longer by turning our attention to abundant numbers. For one thing, they are, well, more abundant than perfect numbers. But more importantly, they yield to a little analytic number theory.

As you may recall, an abundant number is a number whose sum of proper divisors exceeds the number itself:

σ0(N) > N

In more modern terms using Euler's sigma function:

σ(N) > 2N

or

σ(N)/N > 2

Let's define

η(n) = σ(N)/N

If you recall the formula for the sigma function for a power of a prime,

σ (pα) = 1 + p + p2 + p3 + ... + pα
= pα(1 + 1/p + 1/p2 + 1/p3 + ... + 1/pα)

For a positive integer in general:

σ (N) = σ(Πpiαi)
= Πσ(piαi)
= Πpiαiη(piαi)
= N * Πη(piαi)

where

η(pα) = 1 + 1/p + 1/p2 + 1/p3 + ... + 1/pα

This gives us a direct measurement of the ratio of σ(N) to N. This doesn't help us find perfect numbers. For that, we still need to find lucky combinations of factors of the single-prime sigmas. But the formula does limit the conditions under which we find perfect and abundant numbers.

How, you ask? The formulae for σ(pα) and η(pα) are both geometric series. However, the latter series converges (to p/(p-1)) where the former does not. So we know that however large we want to make an αi for a given pi, the prime can contribute no more than p/(p-1) towards the final number's η. For the lowest primes:

 p  η(p)      η(p2)     Lim (η(pi), i -> ∞)
--  --------  --------  --------
 2  1.5       1.75      2
 3  1.333333  1.444444  1.5
 5  1.2       1.24      1.25
 7  1.142857  1.163265  1.166666
11  1.090909  1.099173  1.1
13  1.076923  1.082840  1.083333
17  1.058823  1.062283  1.0625
19  1.052631  1.055401  1.055555
23  1.043478  1.045368  1.045454
29  1.034482  1.035671  1.035714
31  1.032258  1.033298  1.033333
37  1.027027  1.027757  1.027777
41  1.024390  1.024985  1.025

As you can see from the table, the higher the prime is, the less it can contribute towards the η value of a number it divides. Since 2 has the highest η values, nearly all abundant numbers are even. The lowest odd abundant numbers are:

 N    prime factorization
====  ===================
 945  33* 5* 7               σ(945)=1920=945*2.031746...
1575  32* 52* 7
2205  32* 5* 72
2835  34* 5* 7
3465  32* 5* 7* 11
4095  32* 5* 7* 13

As more and more of the lowest primes are excluded, more and more different prime factors are required to reach the magical value of 2:

Exclude       Lowest set of         Smallest number of 
primes        prime factors         different prime
              required              factors required
-----------   ----------------      ----------------  
2             3, 5, 7                     3          1.5*1.25*1.16666=2.1875
2,3           5,7,11,13,17,19,23          7          1.25*1.16666*...*1.04545=2.0376
2,3,5         7 thru 61                  15          1.16666*....*1.01666=2.014718
2,3,5,7       11 thru 113                27          (see note (1) below)
2 thru 11     13 thru 193                40 
2 thru 13     17 thru 311                59
2 thru 17     19 thru 439                81
2 thru 19     23 thru 641               109
2 thru 23     29 thru 859               141
2 thru 29     31 thru 1093              174

An abundant number is guaranteed for the "lowest set" from the table above. However, the larger an exponent used for a given prime, the smaller the incremental effect on the η factor. For the "lowest sets" excluding primes greater than 2, the exponents required are very large, and you can get abundant numbers faster by using more primes. For example, the lowest abundant number not divisible by 2 or 3 is 5,391,411,025 = 52*7*11*13*17*19*23*29. Calculating the lowest abundant number whose lowest prime factor is an even higher prime is an exercise left to the reader.


(1) Mathworld (at http://mathworld.wolfram.com) states that Catalan proved that 26 prime factors were required for an odd perfect number, and that Norton extended this to 27 in 1960. I have access to neither Catalan nor Norton to know what techniques they used. But I begin to see the power of analytic number theory.

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