# Question on Big-O notation

by mnb96
Tags: bigo, notation
 P: 625 Hello, I have a polynomial having the form: $$\beta x^5 + \beta^2 x^7 + \beta^3 x^9 + \ldots = \sum_{n=1}^{+\infty}\beta^n x^{2n+3}$$ How can I express this with Big-O notation? Please, note that I consider β as another variable (independent from x). I already know that if β was a constant I could express the above quantity as $O(x^5)$.
 P: 625 First of all I forgot to mention that $x\geq 0$ and $\beta \in \mathbb{R}$. I will try to apply the definition found in Wikipedia, although that definition refers specifically to functions of one-variable. I am not sure we can use that definition, but I will try. Let's "pretend" that β is a constant and x the variable. We have: $$f(x)=\beta x^5 + \beta^2 x^7 + \beta^3 x^9\ldots$$ and I am interested in studying the behavior for $x\to 0$. We have that: $$|\beta x^5 + \beta^2 x^7 + \beta^3 x^9\ldots | \leq |\beta| x^5 + |\beta^2| x^7 + |\beta^3| x^9\ldots \leq |\beta| x^5 + |\beta^2| x^5 + |\beta^3| x^5 \ldots$$, hence we have $f(x)\in O(r^5)$, as expected. By considering β variable, and x constant we have $f(\beta) \in O(\beta)$. Now what?
 P: 424 Question on Big-O notation So you want x->0 and $\beta \to 0$ right? Your series looks a lot like a geometric series. In fact $$\sum_{i=1}^\infty \beta^nx^{3+2n} = x^3\sum_{i=1}^\infty (\beta x^2)^n$$ For small enough $\beta$ and x you should get a nice closed form from which you can more easily see the series' asymptotic behavior.