Equivalence of Definitions of Hyperbolic Secant

Theorem

The following definitions of the concept of Hyperbolic Secant are equivalent:

Definition 1

The hyperbolic secant function is defined on the complex numbers as:

$\sech: X \to \C$:

$\forall z \in X: \sech z := \dfrac 2 {e^z + e^{-z} }$

where:

$X = \set {z: z \in \C, \ e^z + e^{-z} \ne 0}$

Definition 2

The hyperbolic secant function is defined on the complex numbers as:

$\sech: X \to \C$:

$\forall z \in X: \sech z := \dfrac 1 {\cosh z}$

where:

$\cosh$ is the hyperbolic cosine

$X = \set {z: z \in \C, \ \cosh z \ne 0}$

Proof

\(\ds \forall z \in \set {z \in \C: \ e^z + e^{-z} \ne 0}: \, \)

\(\ds \)

\(\)

\(\ds \frac 2 {e^z + e^{-z} }\)

Definition 1 of Hyperbolic Secant

\(\ds \forall z \in \set {z \in \C: \ \frac {e^z + e^{-z} } 2 \ne 0}: \, \)

\(\ds \)

\(=\)

\(\ds 1 / \frac {e^z + e^{-z} } 2\)

\(\ds \forall z \in \set {z \in \C: \ \cosh z \ne 0}: \, \)

\(\ds \)

\(=\)

\(\ds 1 / \cosh z\)

Definition 2 of Hyperbolic Secant

$\blacksquare$