$\newcommand{\braces}[1]{\left\lbrace\,{#1}\,\right\rbrace} \newcommand{\bracks}[1]{\left\lbrack\,{#1}\,\right\rbrack} \newcommand{\dd}{\mathrm{d}} \newcommand{\ds}[1]{\displaystyle{#1}} \newcommand{\expo}[1]{\,\mathrm{e}^{#1}\,} \newcommand{\ic}{\mathrm{i}} \newcommand{\mc}[1]{\mathcal{#1}} \newcommand{\mrm}[1]{\mathrm{#1}} \newcommand{\pars}[1]{\left(\,{#1}\,\right)} \newcommand{\partiald}[3][]{\frac{\partial^{#1} #2}{\partial #3^{#1}}} \newcommand{\root}[2][]{\,\sqrt[#1]{\,{#2}\,}\,} \newcommand{\totald}[3][]{\frac{\mathrm{d}^{#1} #2}{\mathrm{d} #3^{#1}}} \newcommand{\verts}[1]{\left\vert\,{#1}\,\right\vert}$ \begin {align} & \color {#f00}{ \int_ {0}^{ \infty } \int_ {0}^{ \infty } { \ln\pars { \bracks {x + y} \verts {1 - xy}} \over \pars {1 + x^{2}} \pars {1 +y^{2}}} \, \dd x\, \dd \overbrace { \int_ {0}^{ \infty } \int_ {0}^{ \infty } { \ln\pars {x} \over \pars {1 + x^{2}} \pars {1 + y^{2}}}\, \dd x\, \dd y}^{ \ds {=\ 0}} \\ [5mm] +&\N \int_ {0}^{ \infty } \int_ {0}^{ \infty } { \ln\pars {1 + y/x} \over \pars {1 + x^{2}} \pars {1 + y^{2}}}\, \dd x\, \dd y + \int_ {0}^{ \infty } \int_ {0}^{ \infty } { \ln\pars { \verts {1 - xy}} \over \pars {1 + x^{2}} \pars {1 + y^{2}}}\, \dd x\, \dd y \end {align} En la segunda integral, en el lado derecho, deja $\ds{x\ \mapsto\ 1/x}$ : \begin {align} & \color {#f00}{ \int_ {0}^{ \infty } \int_ {0}^{ \infty } { \ln\pars { \bracks {x + y} \verts {1 - xy}} \over \pars {1 + x^{2}} \pars {1 +y^{2}}} \, \dd x\, \dd y} \\ [5mm] = &\\N- \int_ {0}^{ \infty } \int_ {0}^{ \infty } { \ln\pars {1 + y/x} \over \pars {1 + x^{2}} \pars {1 + y^{2}}}\, \dd x\, \dd y + \int_ {0}^{ \infty } \int_ {0}^{ \infty } { \ln\pars { \verts {1 - y/x}} \over \pars {1 + x^{2}} \pars {1 + y^{2}}}\, \dd x\, \dd y \\ [5mm] = &\\N- \int_ {0}^{ \infty }{1 \over x^{2} + 1} \int_ {0}^{ \infty } { \ln\pars { \verts {1 - y^{2}/x^{2}} \over 1 + y^{2}}\, \dd y\, \dd x \,\,\, \stackrel \mapsto\ y}{=}\,\,\, \int_ {0}^{ \infty }{x \over x^{2} + 1} \int_ {0}^{ \infty } { \ln\pars { \verts {1 - y^{2}}} \over 1 + x^{2}y^{2} {\}, \dd y\, \dd x \\ [5mm] = &\\N- \int_ {0}^{ \infty } \ln\pars { \verts {1 - y^{2}}}\ \overbrace { \int_ {0}^{ \infty } {x \over \pars {x^{2} + 1} \pars {y^{2}x^{2} + 1}}, \dd x} ^{ \ds { \ln\pars {y} \over y^{2} - 1}}\ \, \dd y\ =\ \int_ {0}^{ \infty }{ \ln\pars { \verts {1 - y^{2}}} \ln\pars {y} \over y^{2} - 1}\, \dd y \end {align} Ahora, dividimos la integral a lo largo de $\ds{\pars{0,1}}$ y $\ds{\pars{1,\infty}}$ . Más adelante, hacemos la sustitución $\ds{y \mapsto 1/y}$ en la segunda integral $\pars{~\mbox{along}\ \pars{1,\infty}~}$ : \begin {align} & \color {#f00}{ \int_ {0}^{ \infty } \int_ {0}^{ \infty } { \ln\pars { \bracks {x + y} \verts {1 - xy}} \over \pars {1 + x^{2}} \pars {1 +y^{2}}} \, \dd x\, \dd y} \\ [5mm] = &\\N- \int_ {0}^{1}{ \ln\pars {1 - y^{2}} \ln\pars {y} \over y^{2} - 1}\, \dd y + \int_ {0}^{1}{ \bracks { \ln\pars {1 - y^{2}} - 2 \ln\pars {y}} \ln\pars {y} \over y^{2} - 1}\, \dd y \\ [5mm] = &\\N- 2 \int_ {0}^{1} { \ln\pars {1 - y^{2}} \ln\pars {y} - \ln ^{2} \pars {y} \over y^{2} - 1}\, \dd y \,\,\, \stackrel {y^{2}\ \mapsto\ y}{=}\,\,\, 2 \int_ {0}^{1} { \ln ^{2} \pars {y}/4 - \ln\pars {1 - y} \ln\pars {y}/2 \over 1 - y}\,{1 \over 2}\,y^{-1/2} \dd y \\ [5mm] = &\\\\N- {1 \over 4}\ \underbrace {% \int_ {0}^{1}{y^{-1/2} \ln ^{2} \pars {y} \over 1 - y}\, \dd y} _{ \ds {=\ 14 \zeta\pars {3}}}\ -\ {1 \over 2}\ \underbrace { \int_ {0}^{1}{y^{-1/2} \ln\pars {y} \ln\pars {1 - y} \over 1 - y}\, \dd y}_{ \ds {=\ - \pi ^{2} \ln\pars {2} + 7 \zeta\pars {3}}}\ =\ \color {#f00}{{1 \over 2}\, \pi ^{2} \ln\pars {2}} \end {align}
Las dos últimas integrales pueden reducirse directamente a derivadas de la Función Beta y evaluado en límites adecuados.
1 votos
El valor de la misma parece ser $ \frac{\log 2}{2} \pi^2$ . Una sustitución útil parece ser $x = \tan u, \, y = \tan v$ que convierten la integral en $ \int_{0}^{\frac{\pi}{2}} \int_{0}^{\frac{\pi}{2}} \log \left(\left(\tan u + \tan v\right) \left| 1 - \tan u \tan v \right| \right)\,\mathrm{d}u\,\mathrm{d}v $ .
2 votos
@Sirzh He llegado a la misma conclusión. Con un poco de cálculo, se reduce esencialmente a calcular la integral $$\int_0^{\pi/2} \log \cos \theta \, d\theta = -\frac{\pi}{2}\log 2. $$