Looking for another way to calculate the integral $iint_{D}{sin(x)e^{sin(x)sin(y)}}text{d}A$
$begingroup$
Here, I have a little unpleasant way to calculate the following double integral
$$iint_{D}{sin(x)e^{sin(x)sin(y)}}text{d}A$$
where $D$ is the square area $D={(x,y)inmathbb{R}^2: 0 le x le pi/2, 0 le y le pi/2}$
my attempt:
with the symmetry of the area we know
$$I=iint_{D}{sin(x)e^{sin(x)sin(y)}}text{d}A=iint_{D}{sin(y)e^{sin(x)sin(y)}}text{d}A$$
thus
$$I=frac1{2}iint_{D}{(sin(x)+sin(y))e^{sin(x)sin(y)}}text{d}A$$
with the substitution $u=sin(x)$ and $v=sin(y)$, we have
$$I=frac1{2}iint_{D^{*}}{frac{u+v}{sqrt{(1-u^2)(1-v^2)}}e^{uv}}text{d}A$$
and now the integral area is $D^{*}={(u,v)inmathbb{R}^2: 0 le u le 1, 0 le v le 1}$
notice that the function $frac{u+v}{sqrt{(1-u^2)(1-v^2)}}e^{uv}$ also holds a symmetric form, thus
$$I=iint_{D^{*}cap(ule v)}{frac{u+v}{sqrt{(1-u^2)(1-v^2)}}e^{uv}}text{d}A=iint_{D^{*}cap(uge v)}{frac{u+v}{sqrt{(1-u^2)(1-v^2)}}e^{uv}}text{d}A$$
so I can only foucs on area $D^{*}cap(uge v)$, set another substitution
$$u+v=alpha$$
$$uv=beta$$
under the condition $uge v$, the determinant of Jacobian matrix writes
$$|J(alpha,beta)|=frac1{sqrt{alpha^2-4beta}}$$
and
$$frac1{sqrt{(1-u^2)(1-v^2)}}=frac1{sqrt{(1+beta)^2-alpha^2}}$$
the integral area change to ${(alpha,beta)inmathbb{R}^2: 2sqrt{beta} le alpha le 1+beta, 0 le beta le 1}$, so
$$begin{align}
I&=int_{0}^{1}int_{2sqrt{beta}}^{1+beta}{frac{alpha}{sqrt{((1+beta)^2-alpha^2)(alpha^2-4beta)}}e^{beta}}text{d}alpha text{d}beta
\&=int_{0}^{1}left(-tan^{-1}sqrt{frac{(1+beta)^2-alpha^2}{alpha^2-4beta}}right)biggr|_{alpha=2sqrt{beta}}^{1+beta} e^{beta}text{d}beta
\&=frac{pi}{2}(e-1)
end{align}$$
obviously, I choose an inconvenient way in the second half of this calculation, but I still can not come up with another good method.
thanks in advance for any suggestion!
calculus multivariable-calculus
$endgroup$
add a comment |
$begingroup$
Here, I have a little unpleasant way to calculate the following double integral
$$iint_{D}{sin(x)e^{sin(x)sin(y)}}text{d}A$$
where $D$ is the square area $D={(x,y)inmathbb{R}^2: 0 le x le pi/2, 0 le y le pi/2}$
my attempt:
with the symmetry of the area we know
$$I=iint_{D}{sin(x)e^{sin(x)sin(y)}}text{d}A=iint_{D}{sin(y)e^{sin(x)sin(y)}}text{d}A$$
thus
$$I=frac1{2}iint_{D}{(sin(x)+sin(y))e^{sin(x)sin(y)}}text{d}A$$
with the substitution $u=sin(x)$ and $v=sin(y)$, we have
$$I=frac1{2}iint_{D^{*}}{frac{u+v}{sqrt{(1-u^2)(1-v^2)}}e^{uv}}text{d}A$$
and now the integral area is $D^{*}={(u,v)inmathbb{R}^2: 0 le u le 1, 0 le v le 1}$
notice that the function $frac{u+v}{sqrt{(1-u^2)(1-v^2)}}e^{uv}$ also holds a symmetric form, thus
$$I=iint_{D^{*}cap(ule v)}{frac{u+v}{sqrt{(1-u^2)(1-v^2)}}e^{uv}}text{d}A=iint_{D^{*}cap(uge v)}{frac{u+v}{sqrt{(1-u^2)(1-v^2)}}e^{uv}}text{d}A$$
so I can only foucs on area $D^{*}cap(uge v)$, set another substitution
$$u+v=alpha$$
$$uv=beta$$
under the condition $uge v$, the determinant of Jacobian matrix writes
$$|J(alpha,beta)|=frac1{sqrt{alpha^2-4beta}}$$
and
$$frac1{sqrt{(1-u^2)(1-v^2)}}=frac1{sqrt{(1+beta)^2-alpha^2}}$$
the integral area change to ${(alpha,beta)inmathbb{R}^2: 2sqrt{beta} le alpha le 1+beta, 0 le beta le 1}$, so
$$begin{align}
I&=int_{0}^{1}int_{2sqrt{beta}}^{1+beta}{frac{alpha}{sqrt{((1+beta)^2-alpha^2)(alpha^2-4beta)}}e^{beta}}text{d}alpha text{d}beta
\&=int_{0}^{1}left(-tan^{-1}sqrt{frac{(1+beta)^2-alpha^2}{alpha^2-4beta}}right)biggr|_{alpha=2sqrt{beta}}^{1+beta} e^{beta}text{d}beta
\&=frac{pi}{2}(e-1)
end{align}$$
obviously, I choose an inconvenient way in the second half of this calculation, but I still can not come up with another good method.
thanks in advance for any suggestion!
calculus multivariable-calculus
$endgroup$
add a comment |
$begingroup$
Here, I have a little unpleasant way to calculate the following double integral
$$iint_{D}{sin(x)e^{sin(x)sin(y)}}text{d}A$$
where $D$ is the square area $D={(x,y)inmathbb{R}^2: 0 le x le pi/2, 0 le y le pi/2}$
my attempt:
with the symmetry of the area we know
$$I=iint_{D}{sin(x)e^{sin(x)sin(y)}}text{d}A=iint_{D}{sin(y)e^{sin(x)sin(y)}}text{d}A$$
thus
$$I=frac1{2}iint_{D}{(sin(x)+sin(y))e^{sin(x)sin(y)}}text{d}A$$
with the substitution $u=sin(x)$ and $v=sin(y)$, we have
$$I=frac1{2}iint_{D^{*}}{frac{u+v}{sqrt{(1-u^2)(1-v^2)}}e^{uv}}text{d}A$$
and now the integral area is $D^{*}={(u,v)inmathbb{R}^2: 0 le u le 1, 0 le v le 1}$
notice that the function $frac{u+v}{sqrt{(1-u^2)(1-v^2)}}e^{uv}$ also holds a symmetric form, thus
$$I=iint_{D^{*}cap(ule v)}{frac{u+v}{sqrt{(1-u^2)(1-v^2)}}e^{uv}}text{d}A=iint_{D^{*}cap(uge v)}{frac{u+v}{sqrt{(1-u^2)(1-v^2)}}e^{uv}}text{d}A$$
so I can only foucs on area $D^{*}cap(uge v)$, set another substitution
$$u+v=alpha$$
$$uv=beta$$
under the condition $uge v$, the determinant of Jacobian matrix writes
$$|J(alpha,beta)|=frac1{sqrt{alpha^2-4beta}}$$
and
$$frac1{sqrt{(1-u^2)(1-v^2)}}=frac1{sqrt{(1+beta)^2-alpha^2}}$$
the integral area change to ${(alpha,beta)inmathbb{R}^2: 2sqrt{beta} le alpha le 1+beta, 0 le beta le 1}$, so
$$begin{align}
I&=int_{0}^{1}int_{2sqrt{beta}}^{1+beta}{frac{alpha}{sqrt{((1+beta)^2-alpha^2)(alpha^2-4beta)}}e^{beta}}text{d}alpha text{d}beta
\&=int_{0}^{1}left(-tan^{-1}sqrt{frac{(1+beta)^2-alpha^2}{alpha^2-4beta}}right)biggr|_{alpha=2sqrt{beta}}^{1+beta} e^{beta}text{d}beta
\&=frac{pi}{2}(e-1)
end{align}$$
obviously, I choose an inconvenient way in the second half of this calculation, but I still can not come up with another good method.
thanks in advance for any suggestion!
calculus multivariable-calculus
$endgroup$
Here, I have a little unpleasant way to calculate the following double integral
$$iint_{D}{sin(x)e^{sin(x)sin(y)}}text{d}A$$
where $D$ is the square area $D={(x,y)inmathbb{R}^2: 0 le x le pi/2, 0 le y le pi/2}$
my attempt:
with the symmetry of the area we know
$$I=iint_{D}{sin(x)e^{sin(x)sin(y)}}text{d}A=iint_{D}{sin(y)e^{sin(x)sin(y)}}text{d}A$$
thus
$$I=frac1{2}iint_{D}{(sin(x)+sin(y))e^{sin(x)sin(y)}}text{d}A$$
with the substitution $u=sin(x)$ and $v=sin(y)$, we have
$$I=frac1{2}iint_{D^{*}}{frac{u+v}{sqrt{(1-u^2)(1-v^2)}}e^{uv}}text{d}A$$
and now the integral area is $D^{*}={(u,v)inmathbb{R}^2: 0 le u le 1, 0 le v le 1}$
notice that the function $frac{u+v}{sqrt{(1-u^2)(1-v^2)}}e^{uv}$ also holds a symmetric form, thus
$$I=iint_{D^{*}cap(ule v)}{frac{u+v}{sqrt{(1-u^2)(1-v^2)}}e^{uv}}text{d}A=iint_{D^{*}cap(uge v)}{frac{u+v}{sqrt{(1-u^2)(1-v^2)}}e^{uv}}text{d}A$$
so I can only foucs on area $D^{*}cap(uge v)$, set another substitution
$$u+v=alpha$$
$$uv=beta$$
under the condition $uge v$, the determinant of Jacobian matrix writes
$$|J(alpha,beta)|=frac1{sqrt{alpha^2-4beta}}$$
and
$$frac1{sqrt{(1-u^2)(1-v^2)}}=frac1{sqrt{(1+beta)^2-alpha^2}}$$
the integral area change to ${(alpha,beta)inmathbb{R}^2: 2sqrt{beta} le alpha le 1+beta, 0 le beta le 1}$, so
$$begin{align}
I&=int_{0}^{1}int_{2sqrt{beta}}^{1+beta}{frac{alpha}{sqrt{((1+beta)^2-alpha^2)(alpha^2-4beta)}}e^{beta}}text{d}alpha text{d}beta
\&=int_{0}^{1}left(-tan^{-1}sqrt{frac{(1+beta)^2-alpha^2}{alpha^2-4beta}}right)biggr|_{alpha=2sqrt{beta}}^{1+beta} e^{beta}text{d}beta
\&=frac{pi}{2}(e-1)
end{align}$$
obviously, I choose an inconvenient way in the second half of this calculation, but I still can not come up with another good method.
thanks in advance for any suggestion!
calculus multivariable-calculus
calculus multivariable-calculus
asked Dec 9 '18 at 14:57
NanayajitzukiNanayajitzuki
3185
3185
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1 Answer
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$begingroup$
Using the Taylor expansion of the exponential
$$
iint_{D}{sin(x)e^{sin(x)sin(y)}}text{d}A=sum_{kgeq 0}frac{1}{k!}int_0^{pi/2}mathrm{d}x~(sin x)^{k+1}int_0^{pi/2}mathrm{d}y~(sin y)^{k}=sum_{kgeq 0}frac{1}{k!}I(k)I(k+1) ,
$$
where
$$
I(k)=int_0^{pi/2}mathrm{d}x~(sin x)^{k}=int_0^1mathrm{du}~u^k(1-u^2)^{-1/2}=frac{sqrt{pi}~Gamma left(frac{k+1}{2}right)}{2 Gamma left(frac{k}{2}+1right)} .
$$
Therefore
$$
frac{1}{k!}I(k)I(k+1)=frac{left(sqrt{pi } Gamma left(frac{k+1}{2}right)right) left(sqrt{pi } Gamma left(frac{k}{2}+1right)right)}{k! left(2 Gamma left(frac{k}{2}+1right)right) left(2 Gamma left(frac{k+3}{2}right)right)}=frac{pi }{2 Gamma (k+2)}
$$
after simplifications, which then leads to
$$
sum_{kgeq 0}frac{1}{k!}I(k)I(k+1)=frac{1}{2} (e-1) pi .
$$
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1 Answer
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1 Answer
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$begingroup$
Using the Taylor expansion of the exponential
$$
iint_{D}{sin(x)e^{sin(x)sin(y)}}text{d}A=sum_{kgeq 0}frac{1}{k!}int_0^{pi/2}mathrm{d}x~(sin x)^{k+1}int_0^{pi/2}mathrm{d}y~(sin y)^{k}=sum_{kgeq 0}frac{1}{k!}I(k)I(k+1) ,
$$
where
$$
I(k)=int_0^{pi/2}mathrm{d}x~(sin x)^{k}=int_0^1mathrm{du}~u^k(1-u^2)^{-1/2}=frac{sqrt{pi}~Gamma left(frac{k+1}{2}right)}{2 Gamma left(frac{k}{2}+1right)} .
$$
Therefore
$$
frac{1}{k!}I(k)I(k+1)=frac{left(sqrt{pi } Gamma left(frac{k+1}{2}right)right) left(sqrt{pi } Gamma left(frac{k}{2}+1right)right)}{k! left(2 Gamma left(frac{k}{2}+1right)right) left(2 Gamma left(frac{k+3}{2}right)right)}=frac{pi }{2 Gamma (k+2)}
$$
after simplifications, which then leads to
$$
sum_{kgeq 0}frac{1}{k!}I(k)I(k+1)=frac{1}{2} (e-1) pi .
$$
$endgroup$
add a comment |
$begingroup$
Using the Taylor expansion of the exponential
$$
iint_{D}{sin(x)e^{sin(x)sin(y)}}text{d}A=sum_{kgeq 0}frac{1}{k!}int_0^{pi/2}mathrm{d}x~(sin x)^{k+1}int_0^{pi/2}mathrm{d}y~(sin y)^{k}=sum_{kgeq 0}frac{1}{k!}I(k)I(k+1) ,
$$
where
$$
I(k)=int_0^{pi/2}mathrm{d}x~(sin x)^{k}=int_0^1mathrm{du}~u^k(1-u^2)^{-1/2}=frac{sqrt{pi}~Gamma left(frac{k+1}{2}right)}{2 Gamma left(frac{k}{2}+1right)} .
$$
Therefore
$$
frac{1}{k!}I(k)I(k+1)=frac{left(sqrt{pi } Gamma left(frac{k+1}{2}right)right) left(sqrt{pi } Gamma left(frac{k}{2}+1right)right)}{k! left(2 Gamma left(frac{k}{2}+1right)right) left(2 Gamma left(frac{k+3}{2}right)right)}=frac{pi }{2 Gamma (k+2)}
$$
after simplifications, which then leads to
$$
sum_{kgeq 0}frac{1}{k!}I(k)I(k+1)=frac{1}{2} (e-1) pi .
$$
$endgroup$
add a comment |
$begingroup$
Using the Taylor expansion of the exponential
$$
iint_{D}{sin(x)e^{sin(x)sin(y)}}text{d}A=sum_{kgeq 0}frac{1}{k!}int_0^{pi/2}mathrm{d}x~(sin x)^{k+1}int_0^{pi/2}mathrm{d}y~(sin y)^{k}=sum_{kgeq 0}frac{1}{k!}I(k)I(k+1) ,
$$
where
$$
I(k)=int_0^{pi/2}mathrm{d}x~(sin x)^{k}=int_0^1mathrm{du}~u^k(1-u^2)^{-1/2}=frac{sqrt{pi}~Gamma left(frac{k+1}{2}right)}{2 Gamma left(frac{k}{2}+1right)} .
$$
Therefore
$$
frac{1}{k!}I(k)I(k+1)=frac{left(sqrt{pi } Gamma left(frac{k+1}{2}right)right) left(sqrt{pi } Gamma left(frac{k}{2}+1right)right)}{k! left(2 Gamma left(frac{k}{2}+1right)right) left(2 Gamma left(frac{k+3}{2}right)right)}=frac{pi }{2 Gamma (k+2)}
$$
after simplifications, which then leads to
$$
sum_{kgeq 0}frac{1}{k!}I(k)I(k+1)=frac{1}{2} (e-1) pi .
$$
$endgroup$
Using the Taylor expansion of the exponential
$$
iint_{D}{sin(x)e^{sin(x)sin(y)}}text{d}A=sum_{kgeq 0}frac{1}{k!}int_0^{pi/2}mathrm{d}x~(sin x)^{k+1}int_0^{pi/2}mathrm{d}y~(sin y)^{k}=sum_{kgeq 0}frac{1}{k!}I(k)I(k+1) ,
$$
where
$$
I(k)=int_0^{pi/2}mathrm{d}x~(sin x)^{k}=int_0^1mathrm{du}~u^k(1-u^2)^{-1/2}=frac{sqrt{pi}~Gamma left(frac{k+1}{2}right)}{2 Gamma left(frac{k}{2}+1right)} .
$$
Therefore
$$
frac{1}{k!}I(k)I(k+1)=frac{left(sqrt{pi } Gamma left(frac{k+1}{2}right)right) left(sqrt{pi } Gamma left(frac{k}{2}+1right)right)}{k! left(2 Gamma left(frac{k}{2}+1right)right) left(2 Gamma left(frac{k+3}{2}right)right)}=frac{pi }{2 Gamma (k+2)}
$$
after simplifications, which then leads to
$$
sum_{kgeq 0}frac{1}{k!}I(k)I(k+1)=frac{1}{2} (e-1) pi .
$$
answered Dec 24 '18 at 15:17
Pierpaolo VivoPierpaolo Vivo
5,3562724
5,3562724
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