I have to show using generating functions that decimal expansion of non-negative integer is unique. So I created generating function:
$$ \prod_{k=0}\sum_{i=0}^9 {x^{i \cdot 10^k}} $$
I have to show that coefficient of every power of $x$ is equal to $1$. In other word I have to show that constructed generating function is equal to $\sum_{i=0}x^i$. I am completely helpless maybe there is some simpler way?
Suppose we restrict ourselves to $m+1$-digit integers. Then the generating function becomes $$f_m(x) = \prod_{k=0}^m \sum_{j=0}^9 x^{j 10^k}.$$ Evaluating the sum this is $$f_m(x) = \prod_{k=0}^m \frac{x^{10\times 10^k} - 1}{x^{10^k}-1} = \prod_{k=0}^m \frac{x^{10^{k+1}} - 1}{x^{10^k}-1}.$$ This product telescopes and the only terms left over are $$f_m(x) = \frac{x^{10^{m+1}}-1}{x-1}.$$ But this is a geometric series and $$f_m(x) = \sum_{q=0}^{10^{m+1}-1} x^q.$$ QED.