Primitive of x by Hyperbolic Secant of a x
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Theorem
\(\ds \int x \sech a x \rd x\) | \(=\) | \(\ds \frac 1 {a^2} \sum_{n \mathop = 0}^\infty \frac {E_{2 n} \paren {a x}^{2 n + 2} } {\paren {2 n + 2} \paren {2 n}!} + C\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds \frac 1 {a^2} \paren {\frac {\paren {a x}^2} 2 - \frac {\paren {a x}^4} 8 + \frac {5 \paren {a x}^6} {144} - \cdots } + C\) |
where $E_{2 n}$ denotes the $2 n$th Euler number.
Proof
\(\ds \int x \sech a x \rd x\) | \(=\) | \(\ds \frac 1 {a^2} \int \theta \sech \theta \rd \theta\) | Substitution of $a x \to \theta$ | |||||||||||
\(\ds \) | \(=\) | \(\ds \frac 1 {a^2} \int \theta \sum_{n \mathop = 0}^\infty \frac{E_{2 n} \, \theta^{2 n} } {\paren {2 n}!} \rd \theta\) | Power Series Expansion for Hyperbolic Secant Function | |||||||||||
\(\ds \) | \(=\) | \(\ds \frac 1 {a^2} \sum_{n \mathop = 0}^\infty \frac {E_{2 n} } {\paren {2 n}!} \int \theta^{2 n + 1} \rd \theta\) | Fubini's Theorem | |||||||||||
\(\ds \) | \(=\) | \(\ds \frac 1 {a^2} \sum_{n \mathop = 0}^\infty \frac {E_{2 n} \paren {a x}^{2 n + 2} } {\paren {2 n + 2} \paren {2 n}!} + C\) | Substituting back $\theta \to ax$ |
$\blacksquare$
Also see
- Primitive of $x \sinh a x$
- Primitive of $x \cosh a x$
- Primitive of $x \tanh a x$
- Primitive of $x \coth a x$
- Primitive of $x \csch a x$
Sources
- 1968: Murray R. Spiegel: Mathematical Handbook of Formulas and Tables ... (previous) ... (next): $\S 14$: Integrals involving $\sech a x$: $14.631$