Theorem: If $f$ is a non-negative and non-increasing function on the interval $[1,\infty)$, then the improper integral $$\int_1^\infty f(x)\, dx$$ and the infinite series $$\sum_{n=1}^\infty f(n)$$ either both converge or both diverge.
Corollary ($p$-series): $\displaystyle{\sum_{n=1}^\infty \frac{1}{n^p}}$ converges if $p>1$ and diverges if $p \le 1$.