Question Number 116097 by mathmax by abdo last updated on 30/Sep/20
$$\mathrm{calculate}\:\int_{\mathrm{0}} ^{\infty} \:\frac{\mathrm{lnx}}{\mathrm{x}^{\mathrm{4}} \:+\mathrm{1}}\mathrm{dx}\: \\ $$
Answered by mnjuly1970 last updated on 01/Oct/20
$$\:\:{solution}:\:\:\:\Omega=\int_{\mathrm{0}} ^{\:\infty} \frac{{ln}\left({x}\right)}{\mathrm{1}+{x}^{\mathrm{4}} }\:=??? \\ $$$${x}^{\mathrm{4}} ={t}\:\Rightarrow\:{x}={t}^{\frac{\mathrm{1}}{\mathrm{4}}} \Rightarrow{dx}=\frac{\mathrm{1}}{\mathrm{4}}{t}^{\frac{−\mathrm{3}}{\mathrm{4}}} \\ $$$$\:\Omega\:=\frac{\mathrm{1}}{\mathrm{16}}\int_{\mathrm{0}} ^{\:\infty} \frac{{ln}\left({t}\right){t}^{−\frac{\mathrm{3}}{\mathrm{4}}} }{\mathrm{1}+{t}}{dt} \\ $$$$\:{f}\left(\lambda\right)=\int_{\mathrm{0}\:\:} ^{\:\infty} \frac{{t}^{−\lambda} }{\mathrm{1}+{t}}{dt}\:\Rightarrow\:\:\Omega\:=\frac{−\mathrm{1}}{\mathrm{16}}\:{f}\:'\left(\frac{\mathrm{3}}{\mathrm{4}}\right) \\ $$$${but}::\:\:{f}\left(\lambda\right)\:=\:\Gamma\left(\lambda\right)\Gamma\left(\mathrm{1}−\lambda\right)\:\:;\:\:\:\mathrm{0}<\lambda<\mathrm{1}\: \\ $$$$\:\:\:\:\:\:\:\:\:\:\:\:{f}\left(\lambda\right)=\frac{\pi}{{sin}\left(\lambda\pi\right)}\:\Rightarrow\:{f}\:'\left(\lambda\right)=\left[−\:\frac{\pi^{\mathrm{2}} {cos}\left(\pi\lambda\right)}{{sin}^{\mathrm{2}} \left(\pi\lambda\right)}\right]_{\lambda=\frac{\mathrm{3}}{\mathrm{4}}} \\ $$$$\:\:\:\:\:\:\:\:\:\:\:\:\:\:{f}\:'\left(\frac{\mathrm{3}}{\mathrm{4}}\right)=\pi^{\mathrm{2}} \:\ast\frac{\frac{\sqrt{\mathrm{2}}}{\mathrm{2}}}{\frac{\mathrm{1}}{\mathrm{2}}}\:=\pi^{\mathrm{2}} \:\sqrt{\mathrm{2}} \\ $$$$\:\:\:\:\:\:\Omega:=−\:\frac{\mathrm{1}}{\mathrm{16}}{f}\:'\left(\frac{\mathrm{3}}{\mathrm{4}}\right)=−\frac{\pi^{\mathrm{2}} \:\sqrt{\mathrm{2}}}{\mathrm{16}}\:\:\checkmark \\ $$$$\:\:\:\:\:…{m}.{n}.{july}.\mathrm{1970}… \\ $$$$\:\:\: \\ $$$$\: \\ $$$$\: \\ $$$$ \\ $$$$ \\ $$
Commented by 1549442205PVT last updated on 01/Oct/20
$$\mathrm{third}\:\mathrm{line}\overset{?} {\:\rightarrow}\mathrm{fourth}\:\mathrm{line}\left(\:\mathrm{lnt}\:\mathrm{missed}?\right) \\ $$
Commented by mnjuly1970 last updated on 01/Oct/20
$$\:{method}:\:{feynman}'{s}\:{trick} \\ $$$${please}\:{look}\:{at}\:{again}. \\ $$$$ \\ $$$$\:\:\: \\ $$
Commented by 1549442205PVT last updated on 01/Oct/20
$$\mathrm{Thank}\:\mathrm{Sir},\mathrm{i}\:\mathrm{understanded}.\left(\mathrm{a}^{\mathrm{x}} \right)'=\mathrm{a}^{\mathrm{x}} \mathrm{lna} \\ $$$$\mathrm{and}\:\mathrm{put}\:\mathrm{t}=\frac{\mathrm{1}}{\mathrm{x}}−\mathrm{1}\Rightarrow\mathrm{f}\left(\lambda\right)=\int_{\mathrm{0}} ^{\mathrm{1}} \mathrm{x}^{−\lambda} \left(\mathrm{1}−\mathrm{x}\right)^{\mathrm{1}−\lambda} =\beta\left(\lambda,\mathrm{1}−\lambda\right)=\Gamma\left(\lambda\right)\Gamma\left(\mathrm{1}−\lambda\right)=\frac{\pi}{\mathrm{sin}\left(\lambda\pi\right)} \\ $$$$\mathrm{Great}\:\mathrm{Sir}\:\mathrm{thanh}\:\mathrm{you}\:\mathrm{very}\:\mathrm{much} \\ $$
Commented by mnjuly1970 last updated on 01/Oct/20
$${you}\:{are}\:{welcome}. \\ $$
Commented by Tawa11 last updated on 06/Sep/21
$$\mathrm{grest}\:\mathrm{sir} \\ $$
Answered by mathdave last updated on 01/Oct/20
$${solution} \\ $$$${let}\:{I}=\int_{\mathrm{0}} ^{\mathrm{1}} \frac{\mathrm{ln}{x}}{{x}^{\mathrm{4}} +\mathrm{1}}{dx}+\int_{\mathrm{1}} ^{\infty} \frac{\mathrm{ln}{x}}{\mathrm{1}+{x}^{\mathrm{4}} }{dx}={A}+{B} \\ $$$${putting}\:\:{x}=\frac{\mathrm{1}}{{x}}\:\:\:{into}\:{B} \\ $$$${I}=\int_{\mathrm{0}} ^{\mathrm{1}} \frac{\mathrm{ln}{x}}{\mathrm{1}+{x}^{\mathrm{4}} }{dx}+\int_{\mathrm{1}} ^{\mathrm{0}} \frac{\mathrm{ln}\left(\frac{\mathrm{1}}{{x}}\right)}{\mathrm{1}+\frac{\mathrm{1}}{{x}^{\mathrm{4}} }}×−\frac{\mathrm{1}}{{x}^{\mathrm{2}} }{dx}=\int_{\mathrm{0}} ^{\mathrm{1}} \frac{\mathrm{ln}{x}}{\mathrm{1}+{x}^{\mathrm{4}} }{dx}−\int_{\mathrm{0}} ^{\mathrm{1}} \frac{{x}^{\mathrm{2}} \mathrm{ln}{x}}{\mathrm{1}+{x}^{\mathrm{4}} }{dx} \\ $$$${I}=\left(−\right)^{{n}} \underset{{n}=\mathrm{0}} {\overset{\infty} {\sum}}\int_{\mathrm{0}} ^{\mathrm{1}} {x}^{\mathrm{4}{n}} \mathrm{ln}{xdx}−\left(−\mathrm{1}\right)^{{n}} \underset{{n}=\mathrm{0}} {\overset{\infty} {\sum}}\int_{\mathrm{0}} ^{\mathrm{1}} {x}^{\mathrm{2}} .{x}^{\mathrm{4}{n}} \mathrm{ln}{xdx} \\ $$$${I}=\left(−\mathrm{1}\right)^{{n}} \underset{{n}=\mathrm{0}} {\overset{\infty} {\sum}}\frac{\partial}{\partial{a}}\mid_{{a}=\mathrm{1}} \int_{\mathrm{0}} ^{\mathrm{1}} {x}^{\mathrm{4}{n}} .{x}^{{a}−\mathrm{1}} {dx}−\left(−\mathrm{1}\right)^{{n}} \underset{{n}=\mathrm{0}} {\overset{\infty} {\sum}}\frac{\partial}{\partial{a}}\mid_{{a}=\mathrm{1}} \int_{\mathrm{0}} ^{\mathrm{1}} {x}^{\mathrm{4}{n}+\mathrm{2}} .{x}^{{a}−\mathrm{1}} {dx} \\ $$$${I}=\underset{{n}=\mathrm{0}} {\overset{\infty} {\sum}}\frac{\partial}{\partial{a}}\mid_{{a}=\mathrm{1}} \frac{\left(−\mathrm{1}\right)^{{n}} }{\left(\mathrm{4}{n}+{a}\right)}−\underset{{n}=\mathrm{0}} {\overset{\infty} {\sum}}\frac{\partial}{\partial{a}}\mid_{{a}=\mathrm{1}} \frac{\left(−\mathrm{1}\right)^{{n}} }{\left(\mathrm{4}{n}+\mathrm{2}+{a}\right)} \\ $$$${I}=−\underset{{n}=\mathrm{0}} {\overset{\infty} {\sum}}\frac{\left(−\mathrm{1}\right)^{{n}} }{\left(\mathrm{4}{n}+\mathrm{1}\right)^{\mathrm{2}} }+\underset{{n}=\mathrm{0}} {\overset{\infty} {\sum}}\frac{\left(−\mathrm{1}\right)^{{n}} }{\left(\mathrm{4}{n}+\mathrm{3}\right)^{\mathrm{2}} }=−\underset{{n}=−\infty} {\overset{\infty} {\sum}}\frac{\left(−\mathrm{1}\right)^{{n}} }{\left(\mathrm{4}{n}+\mathrm{1}\right)^{\mathrm{2}} } \\ $$$${note}\:\underset{{n}=−\infty} {\overset{\infty} {\sum}}\frac{\left(−\mathrm{1}\right)^{{n}} }{\left({ax}+\mathrm{1}\right)^{{n}} }=−\frac{\pi}{{a}^{{n}} }\underset{{z}\rightarrow−\frac{\mathrm{1}}{{a}}} {\mathrm{lim}}\frac{\mathrm{1}}{\left({n}−\mathrm{1}\right)!}\bullet\frac{{d}^{{n}−\mathrm{1}} }{{dz}^{{n}−\mathrm{1}} }\left(\mathrm{cosec}\left(\pi{z}\right)\right) \\ $$$${I}=−\underset{{n}=−\infty} {\overset{\infty} {\sum}}\frac{\mathrm{1}}{\left(\mathrm{4}{n}+\mathrm{1}\right)^{\mathrm{2}} }=−\left(−\frac{\pi}{\mathrm{4}^{\mathrm{2}} }\right)\underset{{z}\rightarrow−\frac{\mathrm{1}}{\mathrm{4}}} {\mathrm{lim}}\frac{\mathrm{1}}{\left(\mathrm{2}−\mathrm{1}\right)!}\bullet\frac{{d}}{{dz}}\left({cosec}\left(\pi{z}\right)\right) \\ $$$${I}=\frac{\pi}{\mathrm{16}}\underset{{z}\rightarrow−\frac{\mathrm{1}}{\mathrm{4}}} {\mathrm{lim}}\frac{\mathrm{1}}{\mathrm{1}!}\left(−\pi\mathrm{cosec}\left(\pi{z}\right)\mathrm{cot}\left(\pi{z}\right)\right) \\ $$$${I}=−\frac{\pi^{\mathrm{2}} }{\mathrm{16}}\mathrm{cosec}\left(−\frac{\pi}{\mathrm{4}}\right)\mathrm{cot}\left(−\frac{\pi}{\mathrm{4}}\right)=−\frac{\pi^{\mathrm{2}} }{\mathrm{16}}\left(−\sqrt{\mathrm{2}}\right)\left(−\mathrm{1}\right)=−\frac{\pi^{\mathrm{2}} }{\mathrm{16}}\sqrt{\mathrm{2}} \\ $$$$\because\int_{\mathrm{0}} ^{\infty} \frac{\mathrm{ln}{x}}{\mathrm{1}+{x}^{\mathrm{4}} }{dx}=−\frac{\pi^{\mathrm{2}} }{\mathrm{8}\sqrt{\mathrm{2}}} \\ $$$${by}\:{mathdave}\left(\mathrm{01}/\mathrm{10}/\mathrm{2020}\right) \\ $$
Commented by Tawa11 last updated on 06/Sep/21
$$\mathrm{great}\:\mathrm{sir} \\ $$
Answered by Bird last updated on 01/Oct/20
$${let}\:{f}\left({a}\right)\:=\int_{\mathrm{0}} ^{\infty} \:\frac{{t}^{{a}−\mathrm{1}} }{\mathrm{1}+{t}}\:{dt}\:\:{with}\:\mathrm{0}<{a}<\mathrm{1} \\ $$$${we}\:{have}\:{f}\left({a}\right)\:=\frac{\pi}{{sin}\left(\pi{a}\right)} \\ $$$${and}\:{f}\left({a}\right)\:=\int_{\mathrm{0}} ^{\infty} \:\frac{{e}^{\left({a}−\mathrm{1}\right){lnt}} }{\mathrm{1}+{t}}{dt}\:\Rightarrow \\ $$$${f}^{'} \left({a}\right)\:=\int_{\mathrm{0}} ^{\infty} \frac{{lnt}\:{t}^{{a}−\mathrm{1}} }{\mathrm{1}+{t}}{dt}=\int_{\mathrm{0}} ^{\infty} \:\frac{{t}^{{a}−\mathrm{1}} {lnt}}{\mathrm{1}+{t}}{dt} \\ $$$$\int_{\mathrm{0}} ^{\infty} \frac{{lnx}}{{x}^{\mathrm{4}} +\mathrm{1}}{dx}\:=_{{x}={t}^{\frac{\mathrm{1}}{\mathrm{4}}} } \frac{\mathrm{1}}{\mathrm{4}}\:\int_{\mathrm{0}} ^{\infty} \:\frac{{lnt}}{\mathrm{1}+{t}}.\frac{\mathrm{1}}{\mathrm{4}}{t}^{\frac{\mathrm{1}}{\mathrm{4}}−\mathrm{1}} {dt} \\ $$$$=\frac{\mathrm{1}}{\mathrm{16}}\:\int_{\mathrm{0}} ^{\infty} \:\frac{{t}^{\frac{\mathrm{1}}{\mathrm{4}}−\mathrm{1}} \:{lnt}}{\mathrm{1}+{t}}{dt} \\ $$$$=\frac{\mathrm{1}}{\mathrm{16}}{f}^{'} \left(\frac{\mathrm{1}}{\mathrm{4}}\right)\:{but}\:{f}^{'} \left({a}\right)=−\frac{\pi^{\mathrm{2}} {cos}\left(\pi{a}\right)}{{sin}^{\mathrm{2}} \left(\pi{a}\right)} \\ $$$$\Rightarrow{f}^{'} \left(\frac{\mathrm{1}}{\mathrm{4}}\right)=−\pi^{\mathrm{2}} ×\frac{\frac{\mathrm{1}}{\:\sqrt{\mathrm{2}}}}{\frac{\mathrm{1}}{\mathrm{2}}}\:=\frac{−\mathrm{2}\pi^{\mathrm{2}} }{\:\sqrt{\mathrm{2}}} \\ $$$$=−\sqrt{\mathrm{2}}\pi^{\mathrm{2}\:} \:\Rightarrow \\ $$$$\int_{\mathrm{0}} ^{\infty} \:\:\frac{{lnx}}{\mathrm{1}+{x}^{\mathrm{4}} }{dx}\:=−\frac{\pi^{\mathrm{2}} \sqrt{\mathrm{2}}}{\mathrm{16}} \\ $$
Commented by mnjuly1970 last updated on 01/Oct/20
$${excellent}\:{sir}.. \\ $$
Commented by Bird last updated on 01/Oct/20
$${you}\:{are}\:{welcome} \\ $$