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Question Number 29077 by abdo imad last updated on 04/Feb/18

let give g(x)=∫_0 ^∞   ((arctan(x(1+t^2 )))/(1+t^2 ))dt  find a simple  form of  g^′ (x) without integral.

$${let}\:{give}\:{g}\left({x}\right)=\int_{\mathrm{0}} ^{\infty} \:\:\frac{{arctan}\left({x}\left(\mathrm{1}+{t}^{\mathrm{2}} \right)\right)}{\mathrm{1}+{t}^{\mathrm{2}} }{dt}\:\:{find}\:{a}\:{simple} \\ $$$${form}\:{of}\:\:{g}^{'} \left({x}\right)\:{without}\:{integral}. \\ $$

Commented by abdo imad last updated on 10/Feb/18

g^′ (x)= ∫_0 ^∞     (dt/(1+x^2 (1+t^2 )^2 ))=(1/2) ∫_(−∞) ^(+∞)      (dt/(1+x^2 (1+t^2 )^2 ))  let introduce the complex function  ϕ(z) =(1/(1+x^2 (1+z^2 )^2 ))  poles ofϕ?  we have  ϕ(z)=    (1/((x(1+z^2 )+i)(x(1+z^2 )−i)))  =       (1/((xz^2 +x+i)( xz^2  +x−i)))= (1/(x^2 (z^2  +1 +(i/x))(z^2  +1−(i/x))))  roots of z^2  +1 +(i/x)=0?⇔ z^2  =−1−(i/x)=(i(√(1+(i/x))))^2   ∣1+(i/x)∣=(√(1+(1/x^2 ) ))  =((√(1+x^2 ))/(∣x∣))and 1+(i/x)  =((√(1+x^2 ))/(∣x∣))((1/((√(1+x^2 ))/(∣x∣))) + (1/(x((√(1+x^2 ))/(∣x∣))))i)=r e^(iθ)   ⇒cosθ=((∣x∣)/(√(1+x^2 )))  and sinθ =((∣x∣)/(x(√(1+x^2 )))) .let suppose x>0⇒cosθ= (x/(√(1+x^2 )))  and sinθ= (1/(√(1+x^2 )))⇒tanθ= (1/x) ⇒θ=arctan((1/x))and  1+(i/x)=((√(1+x^2 ))/x) e^(iarctan((1/x)))  ⇒(√(1+(i/x)))  =(((1+x^2 )^(1/4) )/(√x)) e^((i/2)arctan((1/x)))   ⇒  z= +−i (((1+x^2 )^(1/4) )/(√x)) e^((i/2)arctan((1/x)))  the poles are  z_0 =i(((1+x^2 )^(1/4) )/(√x))(cos((1/2)arctan((1/x)) +isin((1/2)arctan((1/x)))  z_1 =−i(((1+x^2 )^(1/4) )/(√x))(cos((1/2)arctan((1/x))+isin((1/2)arctan((1/x)))  roots of z^2  +1 −(i/x)=0   be continued.....

$${g}^{'} \left({x}\right)=\:\int_{\mathrm{0}} ^{\infty} \:\:\:\:\frac{{dt}}{\mathrm{1}+{x}^{\mathrm{2}} \left(\mathrm{1}+{t}^{\mathrm{2}} \right)^{\mathrm{2}} }=\frac{\mathrm{1}}{\mathrm{2}}\:\int_{−\infty} ^{+\infty} \:\:\:\:\:\frac{{dt}}{\mathrm{1}+{x}^{\mathrm{2}} \left(\mathrm{1}+{t}^{\mathrm{2}} \right)^{\mathrm{2}} } \\ $$$${let}\:{introduce}\:{the}\:{complex}\:{function} \\ $$$$\varphi\left({z}\right)\:=\frac{\mathrm{1}}{\mathrm{1}+{x}^{\mathrm{2}} \left(\mathrm{1}+{z}^{\mathrm{2}} \right)^{\mathrm{2}} }\:\:{poles}\:{of}\varphi?\:\:{we}\:{have} \\ $$$$\varphi\left({z}\right)=\:\:\:\:\frac{\mathrm{1}}{\left({x}\left(\mathrm{1}+{z}^{\mathrm{2}} \right)+{i}\right)\left({x}\left(\mathrm{1}+{z}^{\mathrm{2}} \right)−{i}\right)} \\ $$$$=\:\:\:\:\:\:\:\frac{\mathrm{1}}{\left({xz}^{\mathrm{2}} +{x}+{i}\right)\left(\:{xz}^{\mathrm{2}} \:+{x}−{i}\right)}=\:\frac{\mathrm{1}}{{x}^{\mathrm{2}} \left({z}^{\mathrm{2}} \:+\mathrm{1}\:+\frac{{i}}{{x}}\right)\left({z}^{\mathrm{2}} \:+\mathrm{1}−\frac{{i}}{{x}}\right)} \\ $$$${roots}\:{of}\:{z}^{\mathrm{2}} \:+\mathrm{1}\:+\frac{{i}}{{x}}=\mathrm{0}?\Leftrightarrow\:{z}^{\mathrm{2}} \:=−\mathrm{1}−\frac{{i}}{{x}}=\left({i}\sqrt{\mathrm{1}+\frac{{i}}{{x}}}\right)^{\mathrm{2}} \\ $$$$\mid\mathrm{1}+\frac{{i}}{{x}}\mid=\sqrt{\mathrm{1}+\frac{\mathrm{1}}{{x}^{\mathrm{2}} }\:}\:\:=\frac{\sqrt{\mathrm{1}+{x}^{\mathrm{2}} }}{\mid{x}\mid}{and}\:\mathrm{1}+\frac{{i}}{{x}} \\ $$$$=\frac{\sqrt{\mathrm{1}+{x}^{\mathrm{2}} }}{\mid{x}\mid}\left(\frac{\mathrm{1}}{\frac{\sqrt{\mathrm{1}+{x}^{\mathrm{2}} }}{\mid{x}\mid}}\:+\:\frac{\mathrm{1}}{{x}\frac{\sqrt{\mathrm{1}+{x}^{\mathrm{2}} }}{\mid{x}\mid}}{i}\right)={r}\:{e}^{{i}\theta} \:\:\Rightarrow{cos}\theta=\frac{\mid{x}\mid}{\sqrt{\mathrm{1}+{x}^{\mathrm{2}} }} \\ $$$${and}\:{sin}\theta\:=\frac{\mid{x}\mid}{{x}\sqrt{\mathrm{1}+{x}^{\mathrm{2}} }}\:.{let}\:{suppose}\:{x}>\mathrm{0}\Rightarrow{cos}\theta=\:\frac{{x}}{\sqrt{\mathrm{1}+{x}^{\mathrm{2}} }} \\ $$$${and}\:{sin}\theta=\:\frac{\mathrm{1}}{\sqrt{\mathrm{1}+{x}^{\mathrm{2}} }}\Rightarrow{tan}\theta=\:\frac{\mathrm{1}}{{x}}\:\Rightarrow\theta={arctan}\left(\frac{\mathrm{1}}{{x}}\right){and} \\ $$$$\mathrm{1}+\frac{{i}}{{x}}=\frac{\sqrt{\mathrm{1}+{x}^{\mathrm{2}} }}{{x}}\:{e}^{{iarctan}\left(\frac{\mathrm{1}}{{x}}\right)} \:\Rightarrow\sqrt{\mathrm{1}+\frac{{i}}{{x}}}\:\:=\frac{\left(\mathrm{1}+{x}^{\mathrm{2}} \right)^{\frac{\mathrm{1}}{\mathrm{4}}} }{\sqrt{{x}}}\:{e}^{\frac{{i}}{\mathrm{2}}{arctan}\left(\frac{\mathrm{1}}{{x}}\right)} \\ $$$$\Rightarrow\:\:{z}=\:+−{i}\:\frac{\left(\mathrm{1}+{x}^{\mathrm{2}} \right)^{\frac{\mathrm{1}}{\mathrm{4}}} }{\sqrt{{x}}}\:{e}^{\frac{{i}}{\mathrm{2}}{arctan}\left(\frac{\mathrm{1}}{{x}}\right)} \:{the}\:{poles}\:{are} \\ $$$${z}_{\mathrm{0}} ={i}\frac{\left(\mathrm{1}+{x}^{\mathrm{2}} \right)^{\frac{\mathrm{1}}{\mathrm{4}}} }{\sqrt{{x}}}\left({cos}\left(\frac{\mathrm{1}}{\mathrm{2}}{arctan}\left(\frac{\mathrm{1}}{{x}}\right)\:+{isin}\left(\frac{\mathrm{1}}{\mathrm{2}}{arctan}\left(\frac{\mathrm{1}}{{x}}\right)\right)\right.\right. \\ $$$${z}_{\mathrm{1}} =−{i}\frac{\left(\mathrm{1}+{x}^{\mathrm{2}} \right)^{\frac{\mathrm{1}}{\mathrm{4}}} }{\sqrt{{x}}}\left({cos}\left(\frac{\mathrm{1}}{\mathrm{2}}{arctan}\left(\frac{\mathrm{1}}{{x}}\right)+{isin}\left(\frac{\mathrm{1}}{\mathrm{2}}{arctan}\left(\frac{\mathrm{1}}{{x}}\right)\right)\right.\right. \\ $$$${roots}\:{of}\:{z}^{\mathrm{2}} \:+\mathrm{1}\:−\frac{{i}}{{x}}=\mathrm{0}\:\:\:{be}\:{continued}..... \\ $$$$ \\ $$$$ \\ $$$$ \\ $$

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