How do we express $$\displaystyle \int_a^\infty f(x) \, dx \text{ is convergent}$$ using predicate logic?
I know how to express that a simple function is convergent: $$ f(x) \text{ is convergent} $$ means $$ \lim_{x \to \infty} f(x) = L$$ which in turn can be expanded to $$ \exists L \in \mathbb{R}, \forall \epsilon > 0, \exists M \in \mathbb{R}, \forall x \in \mathbb{R}, x \geq M \Longrightarrow | L - f(x) | < \epsilon.$$
Based on that, here is my attempt.
I already know that $$\displaystyle \int_a^\infty f(x) \, dx \text{ is convergent}$$ means $$\lim_{b \to \infty} \int_a^b f(x) \, dx = L.$$
To expand it further, maybe all I have to do is use a similar expression as above, but change $x$ to $b$, and ensure $b > a$, as follows:
$$ \exists L \in \mathbb{R}, \forall \epsilon > 0, \exists M \in \mathbb{R}, \forall b \in \mathbb{R}, ( b \geq M \wedge b > a ) \Longrightarrow \left|L - \int_a^b f(x)\, dx \right| < \epsilon$$
Is this correct?
Your answer is correct. Let $$ \forall x \in \mathbb{R}, \, g(x) = \int_a^x f(t) dt $$ As you noticed \begin{align} \displaystyle \int_a^\infty f(t) \, dt \text{ is convergent} &\iff (\exists L,\lim_{x \rightarrow + \infty} \int_a^x f(t) \, dt =L)\\ &\iff (\exists L,\lim_{x \rightarrow + \infty} g(x) = L) \end{align} Notice that $g$ is just function. Thus, by definition $$ \exists L \in \mathbb{R}, \forall \epsilon > 0, \exists M \in \mathbb{R}, \forall x \in \mathbb{R}, x \geq M \Longrightarrow | L - g(x) | < \epsilon. $$