# Gudermannian

The Gudermannian $\mathrm{gd}$ is defined for $x \in \mathbb{R}$ by the formula $$\mathrm{gd}(x) = \displaystyle\int_0^x \mathrm{sech}(t) \mathrm{d}t,$$ where $\mathrm{sech}$ denotes the hyperbolic secant.
$\ast$-integral functions