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Question Number 57746 by maxmathsup by imad last updated on 10/Apr/19

let f(x)=∫_(−∞) ^(+∞)     (dt/((t^2 −2xt +1)^2 ))  with ∣x∣<1   (x real)  1) determine a explicit form  for f(x)  2) find also g(x) =∫_(−∞) ^(+∞)    ((tdt)/((t^2 −2xt +1)^3 ))  3) calculate ∫_(−∞) ^(+∞)    (dt/((t^2 −(√2)t +1)^2 ))   and ∫_(−∞) ^(+∞)   ((tdt)/((t^2 −(√2)t +1)^3 ))  4) calculate A(θ) =∫_(−∞) ^(+∞)    (dt/((t^2  −2cosθ t+1)^2 ))   and   B(θ) =∫_(−∞) ^(+∞)     ((tdt)/((t^2  −2cosθ t +1)^3 ))    with 0<θ <(π/2)     .

$${let}\:{f}\left({x}\right)=\int_{−\infty} ^{+\infty} \:\:\:\:\frac{{dt}}{\left({t}^{\mathrm{2}} −\mathrm{2}{xt}\:+\mathrm{1}\right)^{\mathrm{2}} }\:\:{with}\:\mid{x}\mid<\mathrm{1}\:\:\:\left({x}\:{real}\right) \\ $$ $$\left.\mathrm{1}\right)\:{determine}\:{a}\:{explicit}\:{form}\:\:{for}\:{f}\left({x}\right) \\ $$ $$\left.\mathrm{2}\right)\:{find}\:{also}\:{g}\left({x}\right)\:=\int_{−\infty} ^{+\infty} \:\:\:\frac{{tdt}}{\left({t}^{\mathrm{2}} −\mathrm{2}{xt}\:+\mathrm{1}\right)^{\mathrm{3}} } \\ $$ $$\left.\mathrm{3}\right)\:{calculate}\:\int_{−\infty} ^{+\infty} \:\:\:\frac{{dt}}{\left({t}^{\mathrm{2}} −\sqrt{\mathrm{2}}{t}\:+\mathrm{1}\right)^{\mathrm{2}} }\:\:\:{and}\:\int_{−\infty} ^{+\infty} \:\:\frac{{tdt}}{\left({t}^{\mathrm{2}} −\sqrt{\mathrm{2}}{t}\:+\mathrm{1}\right)^{\mathrm{3}} } \\ $$ $$\left.\mathrm{4}\right)\:{calculate}\:{A}\left(\theta\right)\:=\int_{−\infty} ^{+\infty} \:\:\:\frac{{dt}}{\left({t}^{\mathrm{2}} \:−\mathrm{2}{cos}\theta\:{t}+\mathrm{1}\right)^{\mathrm{2}} }\:\:\:{and}\: \\ $$ $${B}\left(\theta\right)\:=\int_{−\infty} ^{+\infty} \:\:\:\:\frac{{tdt}}{\left({t}^{\mathrm{2}} \:−\mathrm{2}{cos}\theta\:{t}\:+\mathrm{1}\right)^{\mathrm{3}} }\:\:\:\:{with}\:\mathrm{0}<\theta\:<\frac{\pi}{\mathrm{2}}\:\:\:\:\:. \\ $$

Commented bymaxmathsup by imad last updated on 13/Apr/19

1) let consider the complex function ϕ(z) =(1/((z^2 −2xz +1)^2 ))  let determine the poles of ϕ   z^2 −2xz +1 =0   Δ^′  =x^2 −1 =(i(√(1−x^2 )))^2  ⇒z_1 =x+i(√(1−x^2 )) and z_2 =x−i(√(1−x^2 ))  ϕ(z) =(1/((z−z_1 )^2 (z−z_2 )^2 ))  the poles of ϕ are z_1  and z_2  (doubles)  residus theorem give ∫_(−∞) ^(+∞)  ϕ(z)dz =2iπ Res(ϕ,z_1 )  Res(ϕ,z_1 ) =lim_(z→z_1  )     (1/((2−1)!)){(z−z_1 )^2  ϕ(z)}^((1))   =lim_(z→z_1 )    {(1/((z−z_2 )^2 ))}^((1))  =lim_(z→z_1 )    ((−2(z−z_2 ))/((z−z_2 )^4 )) =lim_(z→z_2 )     ((−2)/((z−z_2 )^3 ))  =((−2)/((z_1 −z_2 )^3 )) =((−2)/((2i(√(1−x^2 )))^3 )) =((−2)/(−8i(1−x^2 )(√(1−x^2 )))) =(1/(4i(1−x^2 )(√(1−x^2 )))) ⇒  f(x) =∫_(−∞) ^(+∞)  ϕ(z)dz = 2iπ (1/(4i(1−x^2 )(√(1−x^2 )))) ⇒f(x) =(π/(2(1−x^2 )(√(1−x^2 ))))  with ∣x∣<1 .

$$\left.\mathrm{1}\right)\:{let}\:{consider}\:{the}\:{complex}\:{function}\:\varphi\left({z}\right)\:=\frac{\mathrm{1}}{\left({z}^{\mathrm{2}} −\mathrm{2}{xz}\:+\mathrm{1}\right)^{\mathrm{2}} } \\ $$ $${let}\:{determine}\:{the}\:{poles}\:{of}\:\varphi\:\:\:{z}^{\mathrm{2}} −\mathrm{2}{xz}\:+\mathrm{1}\:=\mathrm{0}\: \\ $$ $$\Delta^{'} \:={x}^{\mathrm{2}} −\mathrm{1}\:=\left({i}\sqrt{\mathrm{1}−{x}^{\mathrm{2}} }\right)^{\mathrm{2}} \:\Rightarrow{z}_{\mathrm{1}} ={x}+{i}\sqrt{\mathrm{1}−{x}^{\mathrm{2}} }\:{and}\:{z}_{\mathrm{2}} ={x}−{i}\sqrt{\mathrm{1}−{x}^{\mathrm{2}} } \\ $$ $$\varphi\left({z}\right)\:=\frac{\mathrm{1}}{\left({z}−{z}_{\mathrm{1}} \right)^{\mathrm{2}} \left({z}−{z}_{\mathrm{2}} \right)^{\mathrm{2}} }\:\:{the}\:{poles}\:{of}\:\varphi\:{are}\:{z}_{\mathrm{1}} \:{and}\:{z}_{\mathrm{2}} \:\left({doubles}\right) \\ $$ $${residus}\:{theorem}\:{give}\:\int_{−\infty} ^{+\infty} \:\varphi\left({z}\right){dz}\:=\mathrm{2}{i}\pi\:{Res}\left(\varphi,{z}_{\mathrm{1}} \right) \\ $$ $${Res}\left(\varphi,{z}_{\mathrm{1}} \right)\:={lim}_{{z}\rightarrow{z}_{\mathrm{1}} \:} \:\:\:\:\frac{\mathrm{1}}{\left(\mathrm{2}−\mathrm{1}\right)!}\left\{\left({z}−{z}_{\mathrm{1}} \right)^{\mathrm{2}} \:\varphi\left({z}\right)\right\}^{\left(\mathrm{1}\right)} \\ $$ $$={lim}_{{z}\rightarrow{z}_{\mathrm{1}} } \:\:\:\left\{\frac{\mathrm{1}}{\left({z}−{z}_{\mathrm{2}} \right)^{\mathrm{2}} }\right\}^{\left(\mathrm{1}\right)} \:={lim}_{{z}\rightarrow{z}_{\mathrm{1}} } \:\:\:\frac{−\mathrm{2}\left({z}−{z}_{\mathrm{2}} \right)}{\left({z}−{z}_{\mathrm{2}} \right)^{\mathrm{4}} }\:={lim}_{{z}\rightarrow{z}_{\mathrm{2}} } \:\:\:\:\frac{−\mathrm{2}}{\left({z}−{z}_{\mathrm{2}} \right)^{\mathrm{3}} } \\ $$ $$=\frac{−\mathrm{2}}{\left({z}_{\mathrm{1}} −{z}_{\mathrm{2}} \right)^{\mathrm{3}} }\:=\frac{−\mathrm{2}}{\left(\mathrm{2}{i}\sqrt{\mathrm{1}−{x}^{\mathrm{2}} }\right)^{\mathrm{3}} }\:=\frac{−\mathrm{2}}{−\mathrm{8}{i}\left(\mathrm{1}−{x}^{\mathrm{2}} \right)\sqrt{\mathrm{1}−{x}^{\mathrm{2}} }}\:=\frac{\mathrm{1}}{\mathrm{4}{i}\left(\mathrm{1}−{x}^{\mathrm{2}} \right)\sqrt{\mathrm{1}−{x}^{\mathrm{2}} }}\:\Rightarrow \\ $$ $${f}\left({x}\right)\:=\int_{−\infty} ^{+\infty} \:\varphi\left({z}\right){dz}\:=\:\mathrm{2}{i}\pi\:\frac{\mathrm{1}}{\mathrm{4}{i}\left(\mathrm{1}−{x}^{\mathrm{2}} \right)\sqrt{\mathrm{1}−{x}^{\mathrm{2}} }}\:\Rightarrow{f}\left({x}\right)\:=\frac{\pi}{\mathrm{2}\left(\mathrm{1}−{x}^{\mathrm{2}} \right)\sqrt{\mathrm{1}−{x}^{\mathrm{2}} }} \\ $$ $${with}\:\mid{x}\mid<\mathrm{1}\:. \\ $$

Commented bymaxmathsup by imad last updated on 13/Apr/19

2) we have f^′ (x) =∫_(−∞) ^(+∞)   (∂/∂x)((1/((t^2 −2xt +1)^2 )))dt  =∫_(−∞) ^(+∞)    ((2t(t^2 −2xt +1))/((t^2 −2xt +1)^4 )) dt = 2 ∫_(−∞) ^(+∞)   ((t dt)/((t^2 −2xt +1)^3 )) =2g(x) ⇒g(x)=(1/2)f^′ (x)   we have f(x) =(π/(2(1−x^2 )(√(1−x^2 )))) ⇒f(x) =(π/2){(1−x^2 )^(−1) (1−x^2 )^(−(1/2)) } ⇒  f^′ (x) =(π/2){2x (1−x^2 )^(−2) (1−x^2 )^(−(1/2))   +x(1−x^2 )^(−(3/2)) (1−x^2 )^(−1) }  =(π/2){  ((2x)/((1−x^2 )^2 (√(1−x^2 )))) +(x/((1−x^2 )(1−x^2 )(√(1−x^2 ))))}  =((πx)/2){  (2/((1−x^2 )^2 (√(1−x^2 )))) +(x/((1−x^2 )^2 (√(1−x^2 ))))}  =((πx(x+2))/(2(1−x^2 )^2 (√(1−x^2 )))) ⇒g(x) =((πx(x+2))/(4(1−x^2 )^2 (√(1−x^2 )))) .

$$\left.\mathrm{2}\right)\:{we}\:{have}\:{f}^{'} \left({x}\right)\:=\int_{−\infty} ^{+\infty} \:\:\frac{\partial}{\partial{x}}\left(\frac{\mathrm{1}}{\left({t}^{\mathrm{2}} −\mathrm{2}{xt}\:+\mathrm{1}\right)^{\mathrm{2}} }\right){dt} \\ $$ $$=\int_{−\infty} ^{+\infty} \:\:\:\frac{\mathrm{2}{t}\left({t}^{\mathrm{2}} −\mathrm{2}{xt}\:+\mathrm{1}\right)}{\left({t}^{\mathrm{2}} −\mathrm{2}{xt}\:+\mathrm{1}\right)^{\mathrm{4}} }\:{dt}\:=\:\mathrm{2}\:\int_{−\infty} ^{+\infty} \:\:\frac{{t}\:{dt}}{\left({t}^{\mathrm{2}} −\mathrm{2}{xt}\:+\mathrm{1}\right)^{\mathrm{3}} }\:=\mathrm{2}{g}\left({x}\right)\:\Rightarrow{g}\left({x}\right)=\frac{\mathrm{1}}{\mathrm{2}}{f}^{'} \left({x}\right)\: \\ $$ $${we}\:{have}\:{f}\left({x}\right)\:=\frac{\pi}{\mathrm{2}\left(\mathrm{1}−{x}^{\mathrm{2}} \right)\sqrt{\mathrm{1}−{x}^{\mathrm{2}} }}\:\Rightarrow{f}\left({x}\right)\:=\frac{\pi}{\mathrm{2}}\left\{\left(\mathrm{1}−{x}^{\mathrm{2}} \right)^{−\mathrm{1}} \left(\mathrm{1}−{x}^{\mathrm{2}} \right)^{−\frac{\mathrm{1}}{\mathrm{2}}} \right\}\:\Rightarrow \\ $$ $${f}^{'} \left({x}\right)\:=\frac{\pi}{\mathrm{2}}\left\{\mathrm{2}{x}\:\left(\mathrm{1}−{x}^{\mathrm{2}} \right)^{−\mathrm{2}} \left(\mathrm{1}−{x}^{\mathrm{2}} \right)^{−\frac{\mathrm{1}}{\mathrm{2}}} \:\:+{x}\left(\mathrm{1}−{x}^{\mathrm{2}} \right)^{−\frac{\mathrm{3}}{\mathrm{2}}} \left(\mathrm{1}−{x}^{\mathrm{2}} \right)^{−\mathrm{1}} \right\} \\ $$ $$=\frac{\pi}{\mathrm{2}}\left\{\:\:\frac{\mathrm{2}{x}}{\left(\mathrm{1}−{x}^{\mathrm{2}} \right)^{\mathrm{2}} \sqrt{\mathrm{1}−{x}^{\mathrm{2}} }}\:+\frac{{x}}{\left(\mathrm{1}−{x}^{\mathrm{2}} \right)\left(\mathrm{1}−{x}^{\mathrm{2}} \right)\sqrt{\mathrm{1}−{x}^{\mathrm{2}} }}\right\} \\ $$ $$=\frac{\pi{x}}{\mathrm{2}}\left\{\:\:\frac{\mathrm{2}}{\left(\mathrm{1}−{x}^{\mathrm{2}} \right)^{\mathrm{2}} \sqrt{\mathrm{1}−{x}^{\mathrm{2}} }}\:+\frac{{x}}{\left(\mathrm{1}−{x}^{\mathrm{2}} \right)^{\mathrm{2}} \sqrt{\mathrm{1}−{x}^{\mathrm{2}} }}\right\} \\ $$ $$=\frac{\pi{x}\left({x}+\mathrm{2}\right)}{\mathrm{2}\left(\mathrm{1}−{x}^{\mathrm{2}} \right)^{\mathrm{2}} \sqrt{\mathrm{1}−{x}^{\mathrm{2}} }}\:\Rightarrow{g}\left({x}\right)\:=\frac{\pi{x}\left({x}+\mathrm{2}\right)}{\mathrm{4}\left(\mathrm{1}−{x}^{\mathrm{2}} \right)^{\mathrm{2}} \sqrt{\mathrm{1}−{x}^{\mathrm{2}} }}\:. \\ $$ $$ \\ $$

Commented bymaxmathsup by imad last updated on 13/Apr/19

3) ∫_(−∞) ^(+∞)   (dt/((t^2 −(√2)t +1)^2 )) =f(((√2)/2)) = (π/(2(1−(((√2)/2))^2 (√(1−(((√2)/2))^2 ))))  =(π/(2(1−(1/2))(√(1−(1/2))))) =(π/(√(1/2))) =π(√2)  ∫_(−∞) ^(+∞)    ((tdt)/((t^2  −(√2)t +1)^3 )) =g(((√2)/2)) =((π((√2)/2)(((√2)/2) +2))/(4(1−(((√2)/2))^2 )^2 (√(1−(((√2)/2))^2 ))))  =((π(√2)(4+(√2)))/(16(1−(1/2))^2 (√(1−(1/2))))) =((π(√2)(4+(√2)))/(4.(1/(√2)))) =((2π(4+(√2)))/4) =(π/2)(4+(√2)) .

$$\left.\mathrm{3}\right)\:\int_{−\infty} ^{+\infty} \:\:\frac{{dt}}{\left({t}^{\mathrm{2}} −\sqrt{\mathrm{2}}{t}\:+\mathrm{1}\right)^{\mathrm{2}} }\:={f}\left(\frac{\sqrt{\mathrm{2}}}{\mathrm{2}}\right)\:=\:\frac{\pi}{\mathrm{2}\left(\mathrm{1}−\left(\frac{\sqrt{\mathrm{2}}}{\mathrm{2}}\right)^{\mathrm{2}} \sqrt{\mathrm{1}−\left(\frac{\sqrt{\mathrm{2}}}{\mathrm{2}}\right)^{\mathrm{2}} }\right.} \\ $$ $$=\frac{\pi}{\mathrm{2}\left(\mathrm{1}−\frac{\mathrm{1}}{\mathrm{2}}\right)\sqrt{\mathrm{1}−\frac{\mathrm{1}}{\mathrm{2}}}}\:=\frac{\pi}{\sqrt{\frac{\mathrm{1}}{\mathrm{2}}}}\:=\pi\sqrt{\mathrm{2}} \\ $$ $$\int_{−\infty} ^{+\infty} \:\:\:\frac{{tdt}}{\left({t}^{\mathrm{2}} \:−\sqrt{\mathrm{2}}{t}\:+\mathrm{1}\right)^{\mathrm{3}} }\:={g}\left(\frac{\sqrt{\mathrm{2}}}{\mathrm{2}}\right)\:=\frac{\pi\frac{\sqrt{\mathrm{2}}}{\mathrm{2}}\left(\frac{\sqrt{\mathrm{2}}}{\mathrm{2}}\:+\mathrm{2}\right)}{\mathrm{4}\left(\mathrm{1}−\left(\frac{\sqrt{\mathrm{2}}}{\mathrm{2}}\right)^{\mathrm{2}} \right)^{\mathrm{2}} \sqrt{\mathrm{1}−\left(\frac{\sqrt{\mathrm{2}}}{\mathrm{2}}\right)^{\mathrm{2}} }} \\ $$ $$=\frac{\pi\sqrt{\mathrm{2}}\left(\mathrm{4}+\sqrt{\mathrm{2}}\right)}{\mathrm{16}\left(\mathrm{1}−\frac{\mathrm{1}}{\mathrm{2}}\right)^{\mathrm{2}} \sqrt{\mathrm{1}−\frac{\mathrm{1}}{\mathrm{2}}}}\:=\frac{\pi\sqrt{\mathrm{2}}\left(\mathrm{4}+\sqrt{\mathrm{2}}\right)}{\mathrm{4}.\frac{\mathrm{1}}{\sqrt{\mathrm{2}}}}\:=\frac{\mathrm{2}\pi\left(\mathrm{4}+\sqrt{\mathrm{2}}\right)}{\mathrm{4}}\:=\frac{\pi}{\mathrm{2}}\left(\mathrm{4}+\sqrt{\mathrm{2}}\right)\:. \\ $$

Commented bymaxmathsup by imad last updated on 13/Apr/19

4) we have A(θ) =∫_(−∞) ^(+∞)    (dt/((t^2  −2cosθ t +1)^2 )) =f(cosθ)=(π/(2(1−cos^2 θ)(√(1−cos^2 θ))))  =(π/(2sin^2 θ sinθ)) =(π/(2sin^3 θ))  (     0<θ<(π/2))  also we have B(θ) =∫_(−∞) ^(+∞)    ((tdt)/((t^2 −2cosθ t +1)^3 )) ⇒B(θ) =g(cosθ)  =((π cosθ(cosθ +2))/(4(1−cos^2 θ)^2 (√(1−cos^2 θ)))) =((π cosθ(2+cosθ))/(4sin^4 θ sinθ)) =((2π cosθ +π cos^2 θ)/(4 sin^5 θ)) .

$$\left.\mathrm{4}\right)\:{we}\:{have}\:{A}\left(\theta\right)\:=\int_{−\infty} ^{+\infty} \:\:\:\frac{{dt}}{\left({t}^{\mathrm{2}} \:−\mathrm{2}{cos}\theta\:{t}\:+\mathrm{1}\right)^{\mathrm{2}} }\:={f}\left({cos}\theta\right)=\frac{\pi}{\mathrm{2}\left(\mathrm{1}−{cos}^{\mathrm{2}} \theta\right)\sqrt{\mathrm{1}−{cos}^{\mathrm{2}} \theta}} \\ $$ $$=\frac{\pi}{\mathrm{2}{sin}^{\mathrm{2}} \theta\:{sin}\theta}\:=\frac{\pi}{\mathrm{2}{sin}^{\mathrm{3}} \theta}\:\:\left(\:\:\:\:\:\mathrm{0}<\theta<\frac{\pi}{\mathrm{2}}\right) \\ $$ $${also}\:{we}\:{have}\:{B}\left(\theta\right)\:=\int_{−\infty} ^{+\infty} \:\:\:\frac{{tdt}}{\left({t}^{\mathrm{2}} −\mathrm{2}{cos}\theta\:{t}\:+\mathrm{1}\right)^{\mathrm{3}} }\:\Rightarrow{B}\left(\theta\right)\:={g}\left({cos}\theta\right) \\ $$ $$=\frac{\pi\:{cos}\theta\left({cos}\theta\:+\mathrm{2}\right)}{\mathrm{4}\left(\mathrm{1}−{cos}^{\mathrm{2}} \theta\right)^{\mathrm{2}} \sqrt{\mathrm{1}−{cos}^{\mathrm{2}} \theta}}\:=\frac{\pi\:{cos}\theta\left(\mathrm{2}+{cos}\theta\right)}{\mathrm{4}{sin}^{\mathrm{4}} \theta\:{sin}\theta}\:=\frac{\mathrm{2}\pi\:{cos}\theta\:+\pi\:{cos}^{\mathrm{2}} \theta}{\mathrm{4}\:{sin}^{\mathrm{5}} \theta}\:. \\ $$ $$ \\ $$

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