Question Number 72988 by mathmax by abdo last updated on 05/Nov/19
$${calculate}\:{f}\left({x}\right)=\int_{\mathrm{0}} ^{\infty} \:\:\:\:\frac{{e}^{−{xt}^{\mathrm{2}} } }{\mathrm{4}+{t}^{\mathrm{2}} }{dt}\:\:\:{with}\:{x}>\mathrm{0} \\ $$
Commented by mathmax by abdo last updated on 05/Nov/19
$${we}\:{have}\:{f}\left({x}\right)=\int_{\mathrm{0}} ^{\infty} \:\frac{{e}^{−{x}\left({t}^{\mathrm{2}} +\mathrm{4}−\mathrm{4}\right)} }{{t}^{\mathrm{2}} \:+\mathrm{4}}{dt}\:={e}^{\mathrm{4}{x}} \int_{\mathrm{0}} ^{\infty} \:\:\frac{{e}^{−{x}\left({t}^{\mathrm{2}} +\mathrm{4}\right)} }{{t}^{\mathrm{2}} \:+\mathrm{4}}{dt} \\ $$$$={e}^{\mathrm{4}{x}} {w}\left({x}\right)\:{with}\:{w}\left({x}\right)=\int_{\mathrm{0}} ^{\infty} \:\frac{{e}^{−{x}\left({t}^{\mathrm{2}} \:+\mathrm{4}\right)} }{{t}^{\mathrm{2}} \:+\mathrm{4}}{dt}\:\Rightarrow \\ $$$${w}^{'} \left({x}\right)=−\int_{\mathrm{0}} ^{+\infty} \:\:{e}^{−{x}\left({t}^{\mathrm{2}} +\mathrm{4}\right)} {dt}\:=−{e}^{−\mathrm{4}{x}} \:\int_{\mathrm{0}} ^{\infty} \:{e}^{−\left(\sqrt{{x}}{t}\right)^{\mathrm{2}} } {dt} \\ $$$$=_{\sqrt{{x}}{t}\:={u}} \:\:−\:\:{e}^{−\mathrm{4}{x}} \int_{\mathrm{0}} ^{\infty} \:\:{e}^{−{u}^{\mathrm{2}} } \frac{{du}}{\:\sqrt{{x}}}\:=−\frac{{e}^{−\mathrm{4}{x}} }{\:\sqrt{{x}}}×\frac{\sqrt{\pi}}{\mathrm{2}}\:\Rightarrow{w}\left({x}\right)=−\frac{\sqrt{\pi}}{\mathrm{2}}\int_{\mathrm{0}} ^{{x}} \:\:\frac{{e}^{−\mathrm{4}{u}} }{\:\sqrt{{u}}}{du}\:+{c} \\ $$$$=_{\sqrt{{u}}={z}} −\:\:\:\frac{\sqrt{\pi}}{\mathrm{2}}\:\int_{\mathrm{0}} ^{\sqrt{{x}}} \:\:\frac{{e}^{−\mathrm{4}{z}^{\mathrm{2}} } }{{z}}\left(\mathrm{2}{z}\right){dz}+{c}\:=−\sqrt{\pi}\int_{\mathrm{0}} ^{\sqrt{{x}}} \:{e}^{−\mathrm{4}{z}^{\mathrm{2}} } {dz}\:+{c}\Rightarrow \\ $$$${f}\left({x}\right)={c}\:{e}^{\mathrm{4}{x}} \:−\sqrt{\pi}{e}^{\mathrm{4}{x}} \:\int_{\mathrm{0}} ^{\sqrt{{x}}} \:{e}^{−\mathrm{4}{z}^{\mathrm{2}} } {dz} \\ $$$${c}={lim}_{{x}\rightarrow\mathrm{0}} {f}\left({x}\right)\:=\int_{\mathrm{0}} ^{\infty} \:\:\frac{{dt}}{{t}^{\mathrm{2}} \:+\mathrm{4}}\:=_{{t}=\mathrm{2}{u}} \:\:\int_{\mathrm{0}} ^{\infty} \:\:\:\frac{\mathrm{2}{du}}{\mathrm{4}\left(\mathrm{1}+{u}^{\mathrm{2}} \right)}\:=\frac{\mathrm{1}}{\mathrm{2}}\frac{\pi}{\mathrm{2}}=\frac{\pi}{\mathrm{4}}\:\Rightarrow \\ $$$${f}\left({x}\right)=\frac{\pi}{\mathrm{4}}{e}^{\mathrm{4}{x}} \:−\sqrt{\pi}{e}^{\mathrm{4}{x}} \:\int_{\mathrm{0}} ^{\sqrt{{x}}} \:{e}^{−\mathrm{4}{z}^{\mathrm{2}} } {dz} \\ $$$$ \\ $$
Answered by mind is power last updated on 05/Nov/19
$$\mathrm{f}'\left(\mathrm{x}\right)=\int_{\mathrm{0}} ^{+\infty} \frac{−\mathrm{t}^{\mathrm{2}} \mathrm{e}^{−\mathrm{xt}^{\mathrm{2}} } }{\mathrm{4}+\mathrm{t}^{\mathrm{2}} }=\int−\mathrm{e}^{−\mathrm{xt}^{\mathrm{2}} } +\mathrm{4}\int_{\mathrm{0}} ^{+\infty} \frac{\mathrm{e}^{−\mathrm{xt}^{\mathrm{2}} } }{\mathrm{4}+\mathrm{t}^{\mathrm{2}} }\mathrm{dt} \\ $$$$=\Rightarrow\mathrm{f}'\left(\mathrm{x}\right)=\mathrm{4f}\left(\mathrm{x}\right)−\frac{\mathrm{1}}{\:\sqrt{\mathrm{x}}}\int_{\mathrm{0}} ^{+\infty} \mathrm{e}^{−\left(\mathrm{t}\sqrt{\mathrm{x}}\right)^{\mathrm{2}} } .\sqrt{\mathrm{x}}\mathrm{dt} \\ $$$$\Rightarrow\mathrm{f}'\left(\mathrm{x}\right)=\mathrm{4f}\left(\mathrm{x}\right)−\frac{\mathrm{1}}{\:\sqrt{\mathrm{x}}}.\int_{\mathrm{0}} ^{+\infty} \mathrm{e}^{−\mathrm{u}^{\mathrm{2}} } \mathrm{du}=\mathrm{4f}\left(\mathrm{x}\right)−\frac{\mathrm{1}}{\mathrm{2}\sqrt{\mathrm{x}}}.\sqrt{\frac{\pi}{\mathrm{2}}} \\ $$$$\mathrm{f}'\left(\mathrm{x}\right)−\mathrm{4f}\left(\mathrm{x}\right)+\frac{\mathrm{1}}{\mathrm{2}\sqrt{\mathrm{x}}}.\sqrt{\frac{\pi}{\mathrm{2}}}=\mathrm{0} \\ $$$$\mathrm{f}\left(\mathrm{x}\right)=\mathrm{ke}^{\mathrm{4x}} \\ $$$$\Rightarrow\mathrm{k}'\left(\mathrm{x}\right)=−\frac{\mathrm{e}^{−\mathrm{4x}} }{\mathrm{2}\sqrt{\mathrm{x}}}\sqrt{\frac{\pi}{\mathrm{2}}} \\ $$$$=\Rightarrow\mathrm{k}\left(\mathrm{x}\right)=−\frac{\sqrt{\pi}}{\mathrm{2}\sqrt{\mathrm{2}}}\int\frac{\mathrm{e}^{−\mathrm{4x}} }{\mathrm{2}\sqrt{\mathrm{x}}}\mathrm{dx} \\ $$$$\mathrm{erf}\left(\mathrm{x}\right)=\int_{\mathrm{0}} ^{\mathrm{x}} \mathrm{e}^{−\mathrm{t}^{\mathrm{2}} } \mathrm{dt} \\ $$$$\sqrt{\mathrm{x}}=\mathrm{u}\Rightarrow\mathrm{k}\left(\mathrm{x}\right)=−\frac{\sqrt{\pi}}{\mathrm{2}\sqrt{\mathrm{2}}}\int\mathrm{e}^{−\mathrm{4u}^{\mathrm{2}} } \mathrm{du}=−\frac{\sqrt{\pi}}{\mathrm{4}\sqrt{\mathrm{2}}}\int\mathrm{e}^{−\mathrm{w}^{\mathrm{2}} } \mathrm{dw}=−\frac{\sqrt{\pi}}{\mathrm{4}\sqrt{\mathrm{2}}}\mathrm{erf}\left(\mathrm{w}\right) \\ $$$$=\frac{\sqrt{\pi}}{\mathrm{4}\sqrt{\mathrm{2}}}\mathrm{erf}\left(\mathrm{2u}\right)=−\frac{\sqrt{\pi}}{\mathrm{4}\sqrt{\mathrm{2}}}\mathrm{erf}\left(\mathrm{2}\sqrt{\mathrm{x}}\right)+\mathrm{c} \\ $$$$\Rightarrow\mathrm{f}\left(\mathrm{x}\right)=\mathrm{ce}^{−\mathrm{4x}} −\frac{\sqrt{\pi}}{\mathrm{4}\sqrt{\mathrm{2}}}\mathrm{erf}\left(\mathrm{2}\sqrt{\mathrm{x}}\right)\mathrm{e}^{−\mathrm{4x}} \\ $$$$\mathrm{f}\left(\mathrm{0}\right)=\int_{\mathrm{0}} ^{+\infty} \frac{\mathrm{1}}{\mathrm{4}+\mathrm{t}^{\mathrm{2}} }=\left[\frac{\mathrm{1}}{\mathrm{2}}.\mathrm{arctan}\left(\frac{\mathrm{t}}{\mathrm{2}}\right)\right]=\frac{\pi}{\mathrm{4}} \\ $$$$\Rightarrow\mathrm{c}=\frac{\pi}{\mathrm{4}} \\ $$$$\mathrm{f}\left(\mathrm{x}\right)=\frac{\pi}{\mathrm{4}}\mathrm{e}^{−\mathrm{4x}} −\frac{\sqrt{\pi}}{\mathrm{4}\sqrt{\mathrm{2}}}\mathrm{erf}\left(\mathrm{2}\sqrt{\mathrm{x}}\right)\mathrm{e}^{−\mathrm{4x}} \\ $$$$ \\ $$$$ \\ $$