Dirichlet Series Absolute Convergence Lemma

Theorem
Let $\displaystyle f(s)= \sum_{n \mathop = 1}^\infty \frac {a_n} {n^s}$ be a Dirichlet series.

Suppose that $f$ converges absolutely at $s_0 = \sigma_0 + i t_0 \in \C$.

Then $f$ converges absolutely at all points $s = \sigma + i t \in \C$ with $\sigma \geq \sigma_0$.

Proof
Suppose that $f$ converges absolutely at $\sigma_0 + i t_0$.

If $\sigma \ge \sigma_0$, then:

Therefore absolute convergence of $f(s_0)$ directly implies absolute convergence of $f(s)$ for all $s = \sigma + i t$ with $\sigma > \sigma_0$.