Solving a double integral/Finding a normal distribution
$begingroup$
Let $sigma^2,alpha,s,t>0$, $Binmathcal{B}(mathbb{R})$ and $xinmathbb{R}$. Consider following integral
begin{align}frac{1}{sqrt{2pifrac{sigma^2}{2alpha}(1-e^{-alpha t})}sqrt{2pifrac{sigma^2}{2alpha}(1-e^{-alpha s})}}cdot\ int_mathbb{R}bigg[int_BexpBig(&-frac{(z-ye^{-alpha t})^2}{2frac{sigma^2}{2alpha}(1-e^{-2alpha t})}Big)dzbigg]quadexpBig(-frac{(y-xe^{-alpha s})^2}{2frac{sigma^2}{2alpha}(1-e^{-2alpha s})}Big)dyend{align}
I want to find it being equal to
$$frac{1}{sqrt{2pifrac{sigma^2}{2alpha}(1-e^{-2alpha(t+s)})}}int_B expBig(-frac{(z-xe^{-alpha (t+s)})^2}{2frac{sigma^2}{2alpha}(1-e^{-2alpha(t+s)})}Big)dz$$
which is a normal distribution. I think one can integrate over $B-ye^{-alpha t}$ and then integrate by substitution, but I struggle finding the result. Thank you for any help!
real-analysis calculus probability analysis probability-distributions
$endgroup$
add a comment |
$begingroup$
Let $sigma^2,alpha,s,t>0$, $Binmathcal{B}(mathbb{R})$ and $xinmathbb{R}$. Consider following integral
begin{align}frac{1}{sqrt{2pifrac{sigma^2}{2alpha}(1-e^{-alpha t})}sqrt{2pifrac{sigma^2}{2alpha}(1-e^{-alpha s})}}cdot\ int_mathbb{R}bigg[int_BexpBig(&-frac{(z-ye^{-alpha t})^2}{2frac{sigma^2}{2alpha}(1-e^{-2alpha t})}Big)dzbigg]quadexpBig(-frac{(y-xe^{-alpha s})^2}{2frac{sigma^2}{2alpha}(1-e^{-2alpha s})}Big)dyend{align}
I want to find it being equal to
$$frac{1}{sqrt{2pifrac{sigma^2}{2alpha}(1-e^{-2alpha(t+s)})}}int_B expBig(-frac{(z-xe^{-alpha (t+s)})^2}{2frac{sigma^2}{2alpha}(1-e^{-2alpha(t+s)})}Big)dz$$
which is a normal distribution. I think one can integrate over $B-ye^{-alpha t}$ and then integrate by substitution, but I struggle finding the result. Thank you for any help!
real-analysis calculus probability analysis probability-distributions
$endgroup$
$begingroup$
If you change the order of integration and focus on the inner integral (with respect to $y$) you will see that the integrand is $c exp(-(text{quadratic function of $y$}))$, where there will be other terms involving $z$ and constants like $alpha$, $sigma^2$, $t$, $s$. By completing the square in the exponent, you can obtain another Gaussian density that allows you to integrate with respect to $y$ over $mathbb{R}$. What remains should be the last line.
$endgroup$
– angryavian
Dec 20 '18 at 20:24
$begingroup$
I do not really understand. Do you mean I should put the two exponential functions together and then integrate over $B$ with respect to $dy$ ?
$endgroup$
– user628255
Dec 20 '18 at 20:41
$begingroup$
Yes, but integrate over $mathbb{R}$ with respect to $dy$.
$endgroup$
– angryavian
Dec 20 '18 at 21:09
$begingroup$
I do not get any solution... Can you give it a try? And maybe post a picture?
$endgroup$
– user628255
Dec 20 '18 at 21:45
add a comment |
$begingroup$
Let $sigma^2,alpha,s,t>0$, $Binmathcal{B}(mathbb{R})$ and $xinmathbb{R}$. Consider following integral
begin{align}frac{1}{sqrt{2pifrac{sigma^2}{2alpha}(1-e^{-alpha t})}sqrt{2pifrac{sigma^2}{2alpha}(1-e^{-alpha s})}}cdot\ int_mathbb{R}bigg[int_BexpBig(&-frac{(z-ye^{-alpha t})^2}{2frac{sigma^2}{2alpha}(1-e^{-2alpha t})}Big)dzbigg]quadexpBig(-frac{(y-xe^{-alpha s})^2}{2frac{sigma^2}{2alpha}(1-e^{-2alpha s})}Big)dyend{align}
I want to find it being equal to
$$frac{1}{sqrt{2pifrac{sigma^2}{2alpha}(1-e^{-2alpha(t+s)})}}int_B expBig(-frac{(z-xe^{-alpha (t+s)})^2}{2frac{sigma^2}{2alpha}(1-e^{-2alpha(t+s)})}Big)dz$$
which is a normal distribution. I think one can integrate over $B-ye^{-alpha t}$ and then integrate by substitution, but I struggle finding the result. Thank you for any help!
real-analysis calculus probability analysis probability-distributions
$endgroup$
Let $sigma^2,alpha,s,t>0$, $Binmathcal{B}(mathbb{R})$ and $xinmathbb{R}$. Consider following integral
begin{align}frac{1}{sqrt{2pifrac{sigma^2}{2alpha}(1-e^{-alpha t})}sqrt{2pifrac{sigma^2}{2alpha}(1-e^{-alpha s})}}cdot\ int_mathbb{R}bigg[int_BexpBig(&-frac{(z-ye^{-alpha t})^2}{2frac{sigma^2}{2alpha}(1-e^{-2alpha t})}Big)dzbigg]quadexpBig(-frac{(y-xe^{-alpha s})^2}{2frac{sigma^2}{2alpha}(1-e^{-2alpha s})}Big)dyend{align}
I want to find it being equal to
$$frac{1}{sqrt{2pifrac{sigma^2}{2alpha}(1-e^{-2alpha(t+s)})}}int_B expBig(-frac{(z-xe^{-alpha (t+s)})^2}{2frac{sigma^2}{2alpha}(1-e^{-2alpha(t+s)})}Big)dz$$
which is a normal distribution. I think one can integrate over $B-ye^{-alpha t}$ and then integrate by substitution, but I struggle finding the result. Thank you for any help!
real-analysis calculus probability analysis probability-distributions
real-analysis calculus probability analysis probability-distributions
edited Dec 20 '18 at 20:39
user628255
asked Dec 20 '18 at 20:16
user628255user628255
224
224
$begingroup$
If you change the order of integration and focus on the inner integral (with respect to $y$) you will see that the integrand is $c exp(-(text{quadratic function of $y$}))$, where there will be other terms involving $z$ and constants like $alpha$, $sigma^2$, $t$, $s$. By completing the square in the exponent, you can obtain another Gaussian density that allows you to integrate with respect to $y$ over $mathbb{R}$. What remains should be the last line.
$endgroup$
– angryavian
Dec 20 '18 at 20:24
$begingroup$
I do not really understand. Do you mean I should put the two exponential functions together and then integrate over $B$ with respect to $dy$ ?
$endgroup$
– user628255
Dec 20 '18 at 20:41
$begingroup$
Yes, but integrate over $mathbb{R}$ with respect to $dy$.
$endgroup$
– angryavian
Dec 20 '18 at 21:09
$begingroup$
I do not get any solution... Can you give it a try? And maybe post a picture?
$endgroup$
– user628255
Dec 20 '18 at 21:45
add a comment |
$begingroup$
If you change the order of integration and focus on the inner integral (with respect to $y$) you will see that the integrand is $c exp(-(text{quadratic function of $y$}))$, where there will be other terms involving $z$ and constants like $alpha$, $sigma^2$, $t$, $s$. By completing the square in the exponent, you can obtain another Gaussian density that allows you to integrate with respect to $y$ over $mathbb{R}$. What remains should be the last line.
$endgroup$
– angryavian
Dec 20 '18 at 20:24
$begingroup$
I do not really understand. Do you mean I should put the two exponential functions together and then integrate over $B$ with respect to $dy$ ?
$endgroup$
– user628255
Dec 20 '18 at 20:41
$begingroup$
Yes, but integrate over $mathbb{R}$ with respect to $dy$.
$endgroup$
– angryavian
Dec 20 '18 at 21:09
$begingroup$
I do not get any solution... Can you give it a try? And maybe post a picture?
$endgroup$
– user628255
Dec 20 '18 at 21:45
$begingroup$
If you change the order of integration and focus on the inner integral (with respect to $y$) you will see that the integrand is $c exp(-(text{quadratic function of $y$}))$, where there will be other terms involving $z$ and constants like $alpha$, $sigma^2$, $t$, $s$. By completing the square in the exponent, you can obtain another Gaussian density that allows you to integrate with respect to $y$ over $mathbb{R}$. What remains should be the last line.
$endgroup$
– angryavian
Dec 20 '18 at 20:24
$begingroup$
If you change the order of integration and focus on the inner integral (with respect to $y$) you will see that the integrand is $c exp(-(text{quadratic function of $y$}))$, where there will be other terms involving $z$ and constants like $alpha$, $sigma^2$, $t$, $s$. By completing the square in the exponent, you can obtain another Gaussian density that allows you to integrate with respect to $y$ over $mathbb{R}$. What remains should be the last line.
$endgroup$
– angryavian
Dec 20 '18 at 20:24
$begingroup$
I do not really understand. Do you mean I should put the two exponential functions together and then integrate over $B$ with respect to $dy$ ?
$endgroup$
– user628255
Dec 20 '18 at 20:41
$begingroup$
I do not really understand. Do you mean I should put the two exponential functions together and then integrate over $B$ with respect to $dy$ ?
$endgroup$
– user628255
Dec 20 '18 at 20:41
$begingroup$
Yes, but integrate over $mathbb{R}$ with respect to $dy$.
$endgroup$
– angryavian
Dec 20 '18 at 21:09
$begingroup$
Yes, but integrate over $mathbb{R}$ with respect to $dy$.
$endgroup$
– angryavian
Dec 20 '18 at 21:09
$begingroup$
I do not get any solution... Can you give it a try? And maybe post a picture?
$endgroup$
– user628255
Dec 20 '18 at 21:45
$begingroup$
I do not get any solution... Can you give it a try? And maybe post a picture?
$endgroup$
– user628255
Dec 20 '18 at 21:45
add a comment |
1 Answer
1
active
oldest
votes
$begingroup$
Here is an approach that avoids directly manipulating the integrals (which is quite painful).
Let $$Y sim N(xe^{-alpha s}, frac{sigma^2}{2alpha}(1-e^{-2alpha s}).$$
Let the conditional distribution of $Z$ given $Y=y$ be $$N(ye^{-alpha t}, frac{sigma^2}{2alpha} (1-e^{-2 alpha t})).$$
Note that the joint distribution satisfies $f_{Y,Z}(y,z) = f_{Z mid Y=y}(z) cdot f_Y(y)$. This is precisely your integrand. In fact, the double integral is computing $P(Z in B)$.
To compute this, one could instead find the marginal distribution of $Z$. I believe one can show that $(Y,Z)$ is bivariate Gaussian, so $Z$ is also Gaussian. Its expectation is $$E[Z] = E[E[Z mid Y]] = E[Ye^{-alpha t}] = xe^{-alpha(s+t)}$$
and its variance is $$text{Var}(Z) = E[text{Var}(Z mid Y)] + text{Var}(E[Z mid Y]) = frac{sigma^2}{2alpha}(1-e^{-2alpha t}) + e^{-2alpha t} frac{sigma^2}{2alpha}(1-e^{-2alpha s}) = frac{sigma^2}{2alpha}(1-e^{-2alpha (s+t)}).$$
Thus you can write down the density $f_Z$ of $Z$, and have $P(Z in B) = int_B f_Z(z) , dz$. This is the goal integral.
$endgroup$
add a comment |
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$begingroup$
Here is an approach that avoids directly manipulating the integrals (which is quite painful).
Let $$Y sim N(xe^{-alpha s}, frac{sigma^2}{2alpha}(1-e^{-2alpha s}).$$
Let the conditional distribution of $Z$ given $Y=y$ be $$N(ye^{-alpha t}, frac{sigma^2}{2alpha} (1-e^{-2 alpha t})).$$
Note that the joint distribution satisfies $f_{Y,Z}(y,z) = f_{Z mid Y=y}(z) cdot f_Y(y)$. This is precisely your integrand. In fact, the double integral is computing $P(Z in B)$.
To compute this, one could instead find the marginal distribution of $Z$. I believe one can show that $(Y,Z)$ is bivariate Gaussian, so $Z$ is also Gaussian. Its expectation is $$E[Z] = E[E[Z mid Y]] = E[Ye^{-alpha t}] = xe^{-alpha(s+t)}$$
and its variance is $$text{Var}(Z) = E[text{Var}(Z mid Y)] + text{Var}(E[Z mid Y]) = frac{sigma^2}{2alpha}(1-e^{-2alpha t}) + e^{-2alpha t} frac{sigma^2}{2alpha}(1-e^{-2alpha s}) = frac{sigma^2}{2alpha}(1-e^{-2alpha (s+t)}).$$
Thus you can write down the density $f_Z$ of $Z$, and have $P(Z in B) = int_B f_Z(z) , dz$. This is the goal integral.
$endgroup$
add a comment |
$begingroup$
Here is an approach that avoids directly manipulating the integrals (which is quite painful).
Let $$Y sim N(xe^{-alpha s}, frac{sigma^2}{2alpha}(1-e^{-2alpha s}).$$
Let the conditional distribution of $Z$ given $Y=y$ be $$N(ye^{-alpha t}, frac{sigma^2}{2alpha} (1-e^{-2 alpha t})).$$
Note that the joint distribution satisfies $f_{Y,Z}(y,z) = f_{Z mid Y=y}(z) cdot f_Y(y)$. This is precisely your integrand. In fact, the double integral is computing $P(Z in B)$.
To compute this, one could instead find the marginal distribution of $Z$. I believe one can show that $(Y,Z)$ is bivariate Gaussian, so $Z$ is also Gaussian. Its expectation is $$E[Z] = E[E[Z mid Y]] = E[Ye^{-alpha t}] = xe^{-alpha(s+t)}$$
and its variance is $$text{Var}(Z) = E[text{Var}(Z mid Y)] + text{Var}(E[Z mid Y]) = frac{sigma^2}{2alpha}(1-e^{-2alpha t}) + e^{-2alpha t} frac{sigma^2}{2alpha}(1-e^{-2alpha s}) = frac{sigma^2}{2alpha}(1-e^{-2alpha (s+t)}).$$
Thus you can write down the density $f_Z$ of $Z$, and have $P(Z in B) = int_B f_Z(z) , dz$. This is the goal integral.
$endgroup$
add a comment |
$begingroup$
Here is an approach that avoids directly manipulating the integrals (which is quite painful).
Let $$Y sim N(xe^{-alpha s}, frac{sigma^2}{2alpha}(1-e^{-2alpha s}).$$
Let the conditional distribution of $Z$ given $Y=y$ be $$N(ye^{-alpha t}, frac{sigma^2}{2alpha} (1-e^{-2 alpha t})).$$
Note that the joint distribution satisfies $f_{Y,Z}(y,z) = f_{Z mid Y=y}(z) cdot f_Y(y)$. This is precisely your integrand. In fact, the double integral is computing $P(Z in B)$.
To compute this, one could instead find the marginal distribution of $Z$. I believe one can show that $(Y,Z)$ is bivariate Gaussian, so $Z$ is also Gaussian. Its expectation is $$E[Z] = E[E[Z mid Y]] = E[Ye^{-alpha t}] = xe^{-alpha(s+t)}$$
and its variance is $$text{Var}(Z) = E[text{Var}(Z mid Y)] + text{Var}(E[Z mid Y]) = frac{sigma^2}{2alpha}(1-e^{-2alpha t}) + e^{-2alpha t} frac{sigma^2}{2alpha}(1-e^{-2alpha s}) = frac{sigma^2}{2alpha}(1-e^{-2alpha (s+t)}).$$
Thus you can write down the density $f_Z$ of $Z$, and have $P(Z in B) = int_B f_Z(z) , dz$. This is the goal integral.
$endgroup$
Here is an approach that avoids directly manipulating the integrals (which is quite painful).
Let $$Y sim N(xe^{-alpha s}, frac{sigma^2}{2alpha}(1-e^{-2alpha s}).$$
Let the conditional distribution of $Z$ given $Y=y$ be $$N(ye^{-alpha t}, frac{sigma^2}{2alpha} (1-e^{-2 alpha t})).$$
Note that the joint distribution satisfies $f_{Y,Z}(y,z) = f_{Z mid Y=y}(z) cdot f_Y(y)$. This is precisely your integrand. In fact, the double integral is computing $P(Z in B)$.
To compute this, one could instead find the marginal distribution of $Z$. I believe one can show that $(Y,Z)$ is bivariate Gaussian, so $Z$ is also Gaussian. Its expectation is $$E[Z] = E[E[Z mid Y]] = E[Ye^{-alpha t}] = xe^{-alpha(s+t)}$$
and its variance is $$text{Var}(Z) = E[text{Var}(Z mid Y)] + text{Var}(E[Z mid Y]) = frac{sigma^2}{2alpha}(1-e^{-2alpha t}) + e^{-2alpha t} frac{sigma^2}{2alpha}(1-e^{-2alpha s}) = frac{sigma^2}{2alpha}(1-e^{-2alpha (s+t)}).$$
Thus you can write down the density $f_Z$ of $Z$, and have $P(Z in B) = int_B f_Z(z) , dz$. This is the goal integral.
answered Dec 20 '18 at 22:31
angryavianangryavian
40.6k23380
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$begingroup$
If you change the order of integration and focus on the inner integral (with respect to $y$) you will see that the integrand is $c exp(-(text{quadratic function of $y$}))$, where there will be other terms involving $z$ and constants like $alpha$, $sigma^2$, $t$, $s$. By completing the square in the exponent, you can obtain another Gaussian density that allows you to integrate with respect to $y$ over $mathbb{R}$. What remains should be the last line.
$endgroup$
– angryavian
Dec 20 '18 at 20:24
$begingroup$
I do not really understand. Do you mean I should put the two exponential functions together and then integrate over $B$ with respect to $dy$ ?
$endgroup$
– user628255
Dec 20 '18 at 20:41
$begingroup$
Yes, but integrate over $mathbb{R}$ with respect to $dy$.
$endgroup$
– angryavian
Dec 20 '18 at 21:09
$begingroup$
I do not get any solution... Can you give it a try? And maybe post a picture?
$endgroup$
– user628255
Dec 20 '18 at 21:45