Idea behind choosing $\small f(x)$ as $c^{s}x^{p-1} \frac{[\theta(x)]^{p}}{(p-1)!}$ in the proof that $\pi$ is transcendental [closed]

Question

I am going through the article at this link, where the author proves that: "$\pi$ is $\text{transcendental}$ over $\mathbb{Q}$". Although, I understand the proof, I have some doubts.

• At page $6$, the author defines a new function $f(x)$ as $$f(x) = c^{s}x^{p-1} \frac{[\theta(x)]^{p}}{(p-1)!}$$ Can anyone tell me what is the motivation behind defining $f(x)$ in this manner.