Zurück zu Bestimmte Integrale
Betrachte die Formel ∫ − ∞ ∞ cosh u x cosh γ x d x = π γ sec ( u π 2 γ ) {\displaystyle \int _{-\infty }^{\infty }{\frac {\cosh ux}{\cosh \gamma x}}\,dx={\frac {\pi }{\gamma }}\,\sec \left({\frac {u\pi }{2\gamma }}\right)} für u = β ± i a {\displaystyle u=\beta \pm ia} mit | Re ( β ) | + | Im ( a ) | < Re ( γ ) {\displaystyle \left|{\text{Re}}(\beta )\right|+\left|{\text{Im}}(a)\right|<{\text{Re}}(\gamma )} . Wegen cosh ( β + i a ) x + cosh ( β − i a ) x = 2 cosh β x cos a x {\displaystyle \cosh(\beta +ia)x+\cosh(\beta -ia)x=2\cosh \beta x\,\cos ax} ist ∫ − ∞ ∞ cosh β x cos a x cosh γ x d x = π 2 γ sec ( ( β + i a ) π 2 γ ) + π 2 γ sec ( ( β − i a ) π 2 γ ) {\displaystyle \int _{-\infty }^{\infty }{\frac {\cosh \beta x\,\cos ax}{\cosh \gamma x}}\,dx={\frac {\pi }{2\gamma }}\,\sec \left({\frac {(\beta +ia)\pi }{2\gamma }}\right)+{\frac {\pi }{2\gamma }}\,\sec \left({\frac {(\beta -ia)\pi }{2\gamma }}\right)} . Integriere nach β {\displaystyle \beta \,} : ∫ − ∞ ∞ sinh β x x cos a x cosh γ x d x = log tan ( π 4 + π ( β + i a ) 4 γ ) + log tan ( π 4 + π ( β − i a ) 4 γ ) {\displaystyle \int _{-\infty }^{\infty }{\frac {\sinh \beta x}{x}}\,{\frac {\cos ax}{\cosh \gamma x}}\,dx=\log \tan \left({\frac {\pi }{4}}+{\frac {\pi (\beta +ia)}{4\gamma }}\right)+\log \tan \left({\frac {\pi }{4}}+{\frac {\pi (\beta -ia)}{4\gamma }}\right)} Und das lässt sich schreiben als log ( tan ( π 2 + β π 2 γ + i a π 2 γ 2 ) ⋅ tan ( π 2 + β π 2 γ − i a π 2 γ 2 ) ) = log cos ( i a π 2 γ ) − cos ( π 2 + β π 2 γ ) cos ( i a π 2 γ ) + cos ( π 2 + β π 2 γ ) = log cosh a π 2 γ + sin β π 2 γ cosh a π 2 γ − sin β π 2 γ {\displaystyle \log \left(\tan \left({\frac {{\frac {\pi }{2}}+{\frac {\beta \pi }{2\gamma }}+{\frac {ia\pi }{2\gamma }}}{2}}\right)\cdot \tan \left({\frac {{\frac {\pi }{2}}+{\frac {\beta \pi }{2\gamma }}-{\frac {ia\pi }{2\gamma }}}{2}}\right)\right)=\log {\frac {\cos \left({\frac {ia\pi }{2\gamma }}\right)-\cos \left({\frac {\pi }{2}}+{\frac {\beta \pi }{2\gamma }}\right)}{\cos \left({\frac {ia\pi }{2\gamma }}\right)+\cos \left({\frac {\pi }{2}}+{\frac {\beta \pi }{2\gamma }}\right)}}=\log {\frac {\cosh {\frac {a\pi }{2\gamma }}+\sin {\frac {\beta \pi }{2\gamma }}}{\cosh {\frac {a\pi }{2\gamma }}-\sin {\frac {\beta \pi }{2\gamma }}}}} .