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Suppose $n$ is a composite natural number. Then $n$ has unique prime factorization. To count the number of proper divisors, simply take the product of the exponents +1 in the prime factorization.

$$n = \prod_{i = 1}^{n} p_i^{a_i}$$

$$\mbox{proper divisors} = \prod_{i = 1}^{n}(a_i+1)$$

Is 1 counted multiple times by doing this?

For instance, I can choose from $a_0+1$ factors contributed from $p_0$, namely

$ 1, p_0, p_0^2, \dots , p_0^{a_0} $ Don't I count 1 multiple times?

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    Your notation is suspect because $n$ is used both as the left hand side and as the limit of the index. Rarely does $n$ have $n$ distinct prime factors, if that is what you want to convey. – 2017-01-17
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    With this product you count *all* divisors, not only the proper ones. – 2017-01-17

3 Answers 3

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You don't count 1 multiple times:

The divisor 1 is only obtained if you choose the exponent $0$ for every prime factor at the same time and the only way of doing that is by choosing every exponent to be $0$; there is only one way of doing it. Hence you do not double count $1$, nor any other divisor.

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In order to count one, the exponent on each prime factor has to be zero. So you are only counting it once.

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As an example of how this works, consider the factors of $72 = 2^3\cdot 3^2$.

The factors are:

$$\begin {array}{c} 1 & 2 & 4 & 8 \\ 3 & 6 & 12 & 24 \\ 9 & 18 & 36 & 72 \end{array}$$

As you can see in this $(3{+}1)\times (2{+}1) $ array, $1$ only occurs once as the multiplication process produces another prime power factors otherwise.