MHB Determining the convergence or divergence of a sequence using direct comparison

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The discussion centers on determining the convergence of the series $$\sum_{n = 2}^{\infty} \frac{{(\ln(n))^{12}}{n^{\frac{9}{8}}}$$ by comparing it to the divergent series $$\sum_{n = 1}^{\infty} \frac{1}{{n}^{\frac{1}{8}}}$$. The key argument is that if it can be shown that $$\frac{{(\ln(n))^{12}}{n^{\frac{9}{8}}} \ge \frac{1}{{n}^{\frac{1}{8}}}$$, then the original series would also diverge. The discussion highlights that since $\ln n$ grows slower than any positive power of $n$, the series behaves similarly to $$\sum\frac{1}{n^{9/8}}$$, which converges. Thus, proving the inequality will confirm the divergence of the original series.
tmt1
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I have

$$\sum_{n = 2}^{\infty} \frac{{(\ln\left({n}\right)})^{12}}{n^{\frac{9}{8}}}$$

We can compare it to $ \frac{1}{{n}^{\frac{1}{8}}}$. $ \sum_{n = 1}^{\infty} \frac{1}{{n}^{\frac{1}{8}}}$ diverges because $p < 1$ in this case. So, if I can prove that $ \frac{{(\ln\left({n}\right)})^{12}}{n^{\frac{9}{8}}} \ge \frac{1}{{n}^{\frac{1}{8}}}$ then that would mean $\sum_{n = 2}^{\infty} \frac{{\ln\left({n}\right)}^{12}}{n^{\frac{9}{8}}}$ diverges. Or $ \frac{{(\ln\left({n}\right)})^{12}}{n^{\frac{9}{8}}} - \frac{1}{{n}^{\frac{1}{8}}} \ge 0 $.

How can I prove this?
 
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tmt said:
I have

$$\sum_{n = 2}^{\infty} \frac{{(\ln\left({n}\right)})^{12}}{n^{\frac{9}{8}}}$$

We can compare it to $ \frac{1}{{n}^{\frac{1}{8}}}$. $ \sum_{n = 1}^{\infty} \frac{1}{{n}^{\frac{1}{8}}}$ diverges because $p < 1$ in this case. So, if I can prove that $ \frac{{(\ln\left({n}\right)})^{12}}{n^{\frac{9}{8}}} \ge \frac{1}{{n}^{\frac{1}{8}}}$ then that would mean $\sum_{n = 2}^{\infty} \frac{{\ln\left({n}\right)}^{12}}{n^{\frac{9}{8}}}$ diverges. Or $ \frac{{(\ln\left({n}\right)})^{12}}{n^{\frac{9}{8}}} - \frac{1}{{n}^{\frac{1}{8}}} \ge 0 $.

How can I prove this?
The guiding principle is that $\ln n$ increases more slowly than any (positive) power of $n$. So for convergence purposes $\sum\frac{(\ln n)^{12}}{n^{9/8}}$ behaves pretty much like $\sum\frac1{n^{9/8}}$. Can you take it from there?
 
There are probably loads of proofs of this online, but I do not want to cheat. Here is my attempt: Convexity says that $$f(\lambda a + (1-\lambda)b) \leq \lambda f(a) + (1-\lambda) f(b)$$ $$f(b + \lambda(a-b)) \leq f(b) + \lambda (f(a) - f(b))$$ We know from the intermediate value theorem that there exists a ##c \in (b,a)## such that $$\frac{f(a) - f(b)}{a-b} = f'(c).$$ Hence $$f(b + \lambda(a-b)) \leq f(b) + \lambda (a - b) f'(c))$$ $$\frac{f(b + \lambda(a-b)) - f(b)}{\lambda(a-b)}...

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