# Show with generating functions that every positive integer has a unique decimal representation.

Question:

Show with generating functions that every positive integer has a unique decimal representation.

Attempt:

I've come up with the following to represent the ones, tens, hundreds, so on:

$$(1+x^1+x^2+...+x^9)(1+x^{10}+x^{20}+..+x^{90})(1+x^{100}+...+x^{900})$$.

I then get this for my generating function:

$$\frac{(1-x^{10})(1-x^{100})(1-x^{1000})...}{(1-x)(1-x^{10})(1-x^{100})...}$$

Even then, I'm not sure if I'm doing the problem correctly.

Edit:

Just realized that the terms cancel out so I get:

$$\frac{1}{(1-x)}$$

I'm stuck here now. I know I need to get something like $$12 = 1*10^1 + 2*10^0$$, but I don't know how to do that with generating functions.

• Not sure if infinite products is the intended approach, but note that if you continued with your approach, a lot of terms will cancel and you will get $\frac{1}{1-x} = 1 + x + x^2 + \cdots$ as desired. – angryavian Oct 16 '18 at 19:23
• @angryavian Didn't notice that. Now I'm stuck with the decimal representation part. – cosmicbrownie Oct 16 '18 at 19:36
• Then you are done because the coefficient of $x^n$ is $1$ for each nonnegative integer $n$. (If there were multiple decimal representations of $n$, then the coefficient would be $> 1$.) – angryavian Oct 16 '18 at 19:39