An untouchable number is a positive integer that cannot be expressed as the sum of all the proper divisors of any positive integer (including the untouchable number itself). That is, these numbers are not in the image of the aliquot sum function. Their study goes back at least to Abu Mansur al-Baghdadi, who observed that both 2 and 5 are untouchable.

## Examples

For example, the number 4 is not untouchable as it is equal to the sum of the proper divisors of 9: 1 + 3 = 4. The number 5 is untouchable as it is not the sum of the proper divisors of any positive integer: 5 = 1 + 4 is the only way to write 5 as the sum of distinct positive integers including 1, but if 4 divides a number, 2 does also, so 1 + 4 cannot be the sum of all of any number's proper divisors (since the list of factors would have to contain both 4 and 2).

The first few untouchable numbers are:

2, 5, 44, 74, 80, X0, X4, 102, 116, 138, 152, 156, 160, 17X, 186, 188, 19X, 1X4, 1E0, 200, 202, 204, 214, 216, 22X, 230, 232, 240, 246, 270, 29X, 2X0, 2E6, 2EX, 314, 334, 336, 356, 370, 372, 374, 382, 390, 3X0, 3X4, 3XX, 400, 408, 430, 440, 442, 444, 46X, 478, 47X, 4E0, 4E6, 4EX, 506, 510, 516, 524, 526, 530, 53X, 540, 552, 554, 560, 56X, 570, 582, 598, 5X8, 5E0, 608, 624, 626, 628, 62X, 632, 652, 65X, 660, 684, 686, 694, 69X, 6E0, 6E6, 718, 730, 732, 744, 750, 756, 75X, 760, 77X, 790, 7X0, 7X6, 7E6, 7E8, 7EX, 808, 80X, 814, 824, 82X, 834, 840, 850, 85X, 870, 87X, 880, 886, 888, 88X, 896, 8X0, 8E4, 900, 914, 916, 918, 91X, 926, 930, 93X, 942, 944, 954, 970, 978, 986, 990, 992, 9X2, 9X4, 9X6, 9EX, X30, X56, X58, X5X, X6X, X74, X82, X86, XX6, XE6, E04, E10, E40, E4X, E56, E80, E82, E90, EE0, EE2, 1000, ...

## Properties

The number 5 is believed to be the only odd untouchable number, but this has not been proven: it would follow from a slightly stronger version of the Goldbach conjecture, since the sum of the proper divisors of pq (with p, q distinct primes) is 1+p+q. Thus, if a number n can be written as a sum of two distinct primes, then n+1 is not an untouchable number. It is expected that every even number larger than 6 is a sum of two distinct primes, so probably no odd number larger than 7 is an untouchable number, and

${\displaystyle 1=\sigma (2)-2}$ ,

${\displaystyle 3=\sigma (4)-4}$ ,

${\displaystyle 7=\sigma (8)-8}$ , so only 5 can be an odd untouchable number.[1] Thus it appears that besides 2 and 5, all untouchable numbers are composite numbers (since except 2, all even numbers are composite).

No perfect number is untouchable, since, at the very least, it can be expressed as the sum of its own proper divisors (this situation happens at the case for 24). Similarly, none of the amicable numbers or sociable numbers are untouchable.

No generalized repunit numbers in prime base is untouchable, since if p is prime, then the sum of the proper divisors of pn is Rn(p) (the generalized repunit number with length n in base p).

No untouchable number is one more than a prime number, since if p is prime, then the sum of the proper divisors of p2 is p + 1. Also, no untouchable number is three more than a prime number, except 5, since if p is an odd prime then the sum of the proper divisors of 2p is p + 3.

## Infinitude

There are infinitely many untouchable numbers, a fact that was proven by Paul Erdős.

1. The stronger version is obtained by adding to the Goldbach conjecture the further requirement that the two primes be distinct - see Template:MathWorld