Laplace transform of a normal distribution
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Let $X$ be a normal distribution with mean $0$ and variance $sigma^2$. Then the Laplace transform of $X$ should be
$int_0^infty e^{-tx} frac{1}{sqrt{2pisigma^2}}exp{-frac{x^2}{2sigma^2}}dx$
Then I try to do integral on
$int_0^infty exp{-frac{x^2}{2sigma^2}-tx}dx=int_0^infty exp{-frac{1}{2sigma^2}[x^2+2sigma^2tx]}dx$
finishing the square we can get
$int_0^infty exp{-frac{1}{2sigma^2}(x+sigma^2 t)^2}dx$
Then I replace $x+sigma^2 t$ by $u$
$int_{sigma^2t}^infty exp{-frac{1}{2sigma^2}u^2}du$
Then I am stuck at this step.
probability-theory probability-distributions normal-distribution laplace-transform
$endgroup$
add a comment |
$begingroup$
Let $X$ be a normal distribution with mean $0$ and variance $sigma^2$. Then the Laplace transform of $X$ should be
$int_0^infty e^{-tx} frac{1}{sqrt{2pisigma^2}}exp{-frac{x^2}{2sigma^2}}dx$
Then I try to do integral on
$int_0^infty exp{-frac{x^2}{2sigma^2}-tx}dx=int_0^infty exp{-frac{1}{2sigma^2}[x^2+2sigma^2tx]}dx$
finishing the square we can get
$int_0^infty exp{-frac{1}{2sigma^2}(x+sigma^2 t)^2}dx$
Then I replace $x+sigma^2 t$ by $u$
$int_{sigma^2t}^infty exp{-frac{1}{2sigma^2}u^2}du$
Then I am stuck at this step.
probability-theory probability-distributions normal-distribution laplace-transform
$endgroup$
$begingroup$
This is as far as it goes: the answer is in terms of the cumulative normal distribution function.
$endgroup$
– uniquesolution
Sep 18 '17 at 15:53
$begingroup$
finishing the square yields another gaussian
$endgroup$
– alain
Jan 22 '18 at 10:51
add a comment |
$begingroup$
Let $X$ be a normal distribution with mean $0$ and variance $sigma^2$. Then the Laplace transform of $X$ should be
$int_0^infty e^{-tx} frac{1}{sqrt{2pisigma^2}}exp{-frac{x^2}{2sigma^2}}dx$
Then I try to do integral on
$int_0^infty exp{-frac{x^2}{2sigma^2}-tx}dx=int_0^infty exp{-frac{1}{2sigma^2}[x^2+2sigma^2tx]}dx$
finishing the square we can get
$int_0^infty exp{-frac{1}{2sigma^2}(x+sigma^2 t)^2}dx$
Then I replace $x+sigma^2 t$ by $u$
$int_{sigma^2t}^infty exp{-frac{1}{2sigma^2}u^2}du$
Then I am stuck at this step.
probability-theory probability-distributions normal-distribution laplace-transform
$endgroup$
Let $X$ be a normal distribution with mean $0$ and variance $sigma^2$. Then the Laplace transform of $X$ should be
$int_0^infty e^{-tx} frac{1}{sqrt{2pisigma^2}}exp{-frac{x^2}{2sigma^2}}dx$
Then I try to do integral on
$int_0^infty exp{-frac{x^2}{2sigma^2}-tx}dx=int_0^infty exp{-frac{1}{2sigma^2}[x^2+2sigma^2tx]}dx$
finishing the square we can get
$int_0^infty exp{-frac{1}{2sigma^2}(x+sigma^2 t)^2}dx$
Then I replace $x+sigma^2 t$ by $u$
$int_{sigma^2t}^infty exp{-frac{1}{2sigma^2}u^2}du$
Then I am stuck at this step.
probability-theory probability-distributions normal-distribution laplace-transform
probability-theory probability-distributions normal-distribution laplace-transform
asked Sep 18 '17 at 15:42
Kenneth.KKenneth.K
499316
499316
$begingroup$
This is as far as it goes: the answer is in terms of the cumulative normal distribution function.
$endgroup$
– uniquesolution
Sep 18 '17 at 15:53
$begingroup$
finishing the square yields another gaussian
$endgroup$
– alain
Jan 22 '18 at 10:51
add a comment |
$begingroup$
This is as far as it goes: the answer is in terms of the cumulative normal distribution function.
$endgroup$
– uniquesolution
Sep 18 '17 at 15:53
$begingroup$
finishing the square yields another gaussian
$endgroup$
– alain
Jan 22 '18 at 10:51
$begingroup$
This is as far as it goes: the answer is in terms of the cumulative normal distribution function.
$endgroup$
– uniquesolution
Sep 18 '17 at 15:53
$begingroup$
This is as far as it goes: the answer is in terms of the cumulative normal distribution function.
$endgroup$
– uniquesolution
Sep 18 '17 at 15:53
$begingroup$
finishing the square yields another gaussian
$endgroup$
– alain
Jan 22 '18 at 10:51
$begingroup$
finishing the square yields another gaussian
$endgroup$
– alain
Jan 22 '18 at 10:51
add a comment |
1 Answer
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$begingroup$
Well, in the general case you're looking at:
$$mathscr{I}_{spacetext{n}}left(text{m}space,spacetext{s}right):=int_0^text{m}frac{text{n}}{expleft(text{s}cdot x+picdottext{n}^2cdot x^2right)}spacetext{d}xtag1$$
Substitute:
$$text{u}:=frac{2cdotpicdottext{n}^2cdot x+text{s}}{2cdotsqrt{pi}cdottext{n}}tag2$$
So, for the integral we get:
$$mathscr{I}_{spacetext{n}}left(text{m}space,spacetext{s}right):=text{n}cdotfrac{expleft(frac{text{s}^2}{4cdotpicdottext{n}^2}right)}{2cdottext{n}}int_{frac{text{s}}{2cdotsqrt{pi}cdottext{n}}}^{frac{2cdotpicdottext{n}^2cdottext{m}+text{s}}{2cdotsqrt{pi}cdottext{n}}}frac{2cdotexpleft(-text{u}^2right)}{sqrt{pi}}spacetext{d}text{u}tag2$$
This is a special integral:
$$int_{frac{text{s}}{2cdotsqrt{pi}cdottext{n}}}^{frac{2cdotpicdottext{n}^2cdottext{m}+text{s}}{2cdotsqrt{pi}cdottext{n}}}frac{2cdotexpleft(-text{u}^2right)}{sqrt{pi}}spacetext{d}text{u}=left[text{erf}left(text{u}right)right]_{frac{text{s}}{2cdotsqrt{pi}cdottext{n}}}^{frac{2cdotpicdottext{n}^2cdottext{m}+text{s}}{2cdotsqrt{pi}cdottext{n}}}=$$
$$text{erf}left(frac{2cdotpicdottext{n}^2cdottext{m}+text{s}}{2cdotsqrt{pi}cdottext{n}}right)-text{erf}left(frac{text{s}}{2cdotsqrt{pi}cdottext{n}}right)tag3$$
So, we end up with:
$$mathscr{I}_{spacetext{n}}left(text{m}space,spacetext{s}right)=text{n}cdotfrac{expleft(frac{text{s}^2}{4cdotpicdottext{n}^2}right)}{2cdottext{n}}cdotleft{text{erf}left(frac{2cdotpicdottext{n}^2cdottext{m}+text{s}}{2cdotsqrt{pi}cdottext{n}}right)-text{erf}left(frac{text{s}}{2cdotsqrt{pi}cdottext{n}}right)right}tag4$$
Now, your task is to simplify and to take:
$$lim_{text{m}toinfty}mathscr{I}_{spacetext{n}}left(text{m}space,spacetext{s}right)tag5$$
Try to prove:
$$lim_{text{m}toinfty}text{erf}left(frac{2cdotpicdottext{n}^2cdottext{m}+text{s}}{2cdotsqrt{pi}cdottext{n}}right)=1tag6$$
$endgroup$
add a comment |
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1 Answer
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$begingroup$
Well, in the general case you're looking at:
$$mathscr{I}_{spacetext{n}}left(text{m}space,spacetext{s}right):=int_0^text{m}frac{text{n}}{expleft(text{s}cdot x+picdottext{n}^2cdot x^2right)}spacetext{d}xtag1$$
Substitute:
$$text{u}:=frac{2cdotpicdottext{n}^2cdot x+text{s}}{2cdotsqrt{pi}cdottext{n}}tag2$$
So, for the integral we get:
$$mathscr{I}_{spacetext{n}}left(text{m}space,spacetext{s}right):=text{n}cdotfrac{expleft(frac{text{s}^2}{4cdotpicdottext{n}^2}right)}{2cdottext{n}}int_{frac{text{s}}{2cdotsqrt{pi}cdottext{n}}}^{frac{2cdotpicdottext{n}^2cdottext{m}+text{s}}{2cdotsqrt{pi}cdottext{n}}}frac{2cdotexpleft(-text{u}^2right)}{sqrt{pi}}spacetext{d}text{u}tag2$$
This is a special integral:
$$int_{frac{text{s}}{2cdotsqrt{pi}cdottext{n}}}^{frac{2cdotpicdottext{n}^2cdottext{m}+text{s}}{2cdotsqrt{pi}cdottext{n}}}frac{2cdotexpleft(-text{u}^2right)}{sqrt{pi}}spacetext{d}text{u}=left[text{erf}left(text{u}right)right]_{frac{text{s}}{2cdotsqrt{pi}cdottext{n}}}^{frac{2cdotpicdottext{n}^2cdottext{m}+text{s}}{2cdotsqrt{pi}cdottext{n}}}=$$
$$text{erf}left(frac{2cdotpicdottext{n}^2cdottext{m}+text{s}}{2cdotsqrt{pi}cdottext{n}}right)-text{erf}left(frac{text{s}}{2cdotsqrt{pi}cdottext{n}}right)tag3$$
So, we end up with:
$$mathscr{I}_{spacetext{n}}left(text{m}space,spacetext{s}right)=text{n}cdotfrac{expleft(frac{text{s}^2}{4cdotpicdottext{n}^2}right)}{2cdottext{n}}cdotleft{text{erf}left(frac{2cdotpicdottext{n}^2cdottext{m}+text{s}}{2cdotsqrt{pi}cdottext{n}}right)-text{erf}left(frac{text{s}}{2cdotsqrt{pi}cdottext{n}}right)right}tag4$$
Now, your task is to simplify and to take:
$$lim_{text{m}toinfty}mathscr{I}_{spacetext{n}}left(text{m}space,spacetext{s}right)tag5$$
Try to prove:
$$lim_{text{m}toinfty}text{erf}left(frac{2cdotpicdottext{n}^2cdottext{m}+text{s}}{2cdotsqrt{pi}cdottext{n}}right)=1tag6$$
$endgroup$
add a comment |
$begingroup$
Well, in the general case you're looking at:
$$mathscr{I}_{spacetext{n}}left(text{m}space,spacetext{s}right):=int_0^text{m}frac{text{n}}{expleft(text{s}cdot x+picdottext{n}^2cdot x^2right)}spacetext{d}xtag1$$
Substitute:
$$text{u}:=frac{2cdotpicdottext{n}^2cdot x+text{s}}{2cdotsqrt{pi}cdottext{n}}tag2$$
So, for the integral we get:
$$mathscr{I}_{spacetext{n}}left(text{m}space,spacetext{s}right):=text{n}cdotfrac{expleft(frac{text{s}^2}{4cdotpicdottext{n}^2}right)}{2cdottext{n}}int_{frac{text{s}}{2cdotsqrt{pi}cdottext{n}}}^{frac{2cdotpicdottext{n}^2cdottext{m}+text{s}}{2cdotsqrt{pi}cdottext{n}}}frac{2cdotexpleft(-text{u}^2right)}{sqrt{pi}}spacetext{d}text{u}tag2$$
This is a special integral:
$$int_{frac{text{s}}{2cdotsqrt{pi}cdottext{n}}}^{frac{2cdotpicdottext{n}^2cdottext{m}+text{s}}{2cdotsqrt{pi}cdottext{n}}}frac{2cdotexpleft(-text{u}^2right)}{sqrt{pi}}spacetext{d}text{u}=left[text{erf}left(text{u}right)right]_{frac{text{s}}{2cdotsqrt{pi}cdottext{n}}}^{frac{2cdotpicdottext{n}^2cdottext{m}+text{s}}{2cdotsqrt{pi}cdottext{n}}}=$$
$$text{erf}left(frac{2cdotpicdottext{n}^2cdottext{m}+text{s}}{2cdotsqrt{pi}cdottext{n}}right)-text{erf}left(frac{text{s}}{2cdotsqrt{pi}cdottext{n}}right)tag3$$
So, we end up with:
$$mathscr{I}_{spacetext{n}}left(text{m}space,spacetext{s}right)=text{n}cdotfrac{expleft(frac{text{s}^2}{4cdotpicdottext{n}^2}right)}{2cdottext{n}}cdotleft{text{erf}left(frac{2cdotpicdottext{n}^2cdottext{m}+text{s}}{2cdotsqrt{pi}cdottext{n}}right)-text{erf}left(frac{text{s}}{2cdotsqrt{pi}cdottext{n}}right)right}tag4$$
Now, your task is to simplify and to take:
$$lim_{text{m}toinfty}mathscr{I}_{spacetext{n}}left(text{m}space,spacetext{s}right)tag5$$
Try to prove:
$$lim_{text{m}toinfty}text{erf}left(frac{2cdotpicdottext{n}^2cdottext{m}+text{s}}{2cdotsqrt{pi}cdottext{n}}right)=1tag6$$
$endgroup$
add a comment |
$begingroup$
Well, in the general case you're looking at:
$$mathscr{I}_{spacetext{n}}left(text{m}space,spacetext{s}right):=int_0^text{m}frac{text{n}}{expleft(text{s}cdot x+picdottext{n}^2cdot x^2right)}spacetext{d}xtag1$$
Substitute:
$$text{u}:=frac{2cdotpicdottext{n}^2cdot x+text{s}}{2cdotsqrt{pi}cdottext{n}}tag2$$
So, for the integral we get:
$$mathscr{I}_{spacetext{n}}left(text{m}space,spacetext{s}right):=text{n}cdotfrac{expleft(frac{text{s}^2}{4cdotpicdottext{n}^2}right)}{2cdottext{n}}int_{frac{text{s}}{2cdotsqrt{pi}cdottext{n}}}^{frac{2cdotpicdottext{n}^2cdottext{m}+text{s}}{2cdotsqrt{pi}cdottext{n}}}frac{2cdotexpleft(-text{u}^2right)}{sqrt{pi}}spacetext{d}text{u}tag2$$
This is a special integral:
$$int_{frac{text{s}}{2cdotsqrt{pi}cdottext{n}}}^{frac{2cdotpicdottext{n}^2cdottext{m}+text{s}}{2cdotsqrt{pi}cdottext{n}}}frac{2cdotexpleft(-text{u}^2right)}{sqrt{pi}}spacetext{d}text{u}=left[text{erf}left(text{u}right)right]_{frac{text{s}}{2cdotsqrt{pi}cdottext{n}}}^{frac{2cdotpicdottext{n}^2cdottext{m}+text{s}}{2cdotsqrt{pi}cdottext{n}}}=$$
$$text{erf}left(frac{2cdotpicdottext{n}^2cdottext{m}+text{s}}{2cdotsqrt{pi}cdottext{n}}right)-text{erf}left(frac{text{s}}{2cdotsqrt{pi}cdottext{n}}right)tag3$$
So, we end up with:
$$mathscr{I}_{spacetext{n}}left(text{m}space,spacetext{s}right)=text{n}cdotfrac{expleft(frac{text{s}^2}{4cdotpicdottext{n}^2}right)}{2cdottext{n}}cdotleft{text{erf}left(frac{2cdotpicdottext{n}^2cdottext{m}+text{s}}{2cdotsqrt{pi}cdottext{n}}right)-text{erf}left(frac{text{s}}{2cdotsqrt{pi}cdottext{n}}right)right}tag4$$
Now, your task is to simplify and to take:
$$lim_{text{m}toinfty}mathscr{I}_{spacetext{n}}left(text{m}space,spacetext{s}right)tag5$$
Try to prove:
$$lim_{text{m}toinfty}text{erf}left(frac{2cdotpicdottext{n}^2cdottext{m}+text{s}}{2cdotsqrt{pi}cdottext{n}}right)=1tag6$$
$endgroup$
Well, in the general case you're looking at:
$$mathscr{I}_{spacetext{n}}left(text{m}space,spacetext{s}right):=int_0^text{m}frac{text{n}}{expleft(text{s}cdot x+picdottext{n}^2cdot x^2right)}spacetext{d}xtag1$$
Substitute:
$$text{u}:=frac{2cdotpicdottext{n}^2cdot x+text{s}}{2cdotsqrt{pi}cdottext{n}}tag2$$
So, for the integral we get:
$$mathscr{I}_{spacetext{n}}left(text{m}space,spacetext{s}right):=text{n}cdotfrac{expleft(frac{text{s}^2}{4cdotpicdottext{n}^2}right)}{2cdottext{n}}int_{frac{text{s}}{2cdotsqrt{pi}cdottext{n}}}^{frac{2cdotpicdottext{n}^2cdottext{m}+text{s}}{2cdotsqrt{pi}cdottext{n}}}frac{2cdotexpleft(-text{u}^2right)}{sqrt{pi}}spacetext{d}text{u}tag2$$
This is a special integral:
$$int_{frac{text{s}}{2cdotsqrt{pi}cdottext{n}}}^{frac{2cdotpicdottext{n}^2cdottext{m}+text{s}}{2cdotsqrt{pi}cdottext{n}}}frac{2cdotexpleft(-text{u}^2right)}{sqrt{pi}}spacetext{d}text{u}=left[text{erf}left(text{u}right)right]_{frac{text{s}}{2cdotsqrt{pi}cdottext{n}}}^{frac{2cdotpicdottext{n}^2cdottext{m}+text{s}}{2cdotsqrt{pi}cdottext{n}}}=$$
$$text{erf}left(frac{2cdotpicdottext{n}^2cdottext{m}+text{s}}{2cdotsqrt{pi}cdottext{n}}right)-text{erf}left(frac{text{s}}{2cdotsqrt{pi}cdottext{n}}right)tag3$$
So, we end up with:
$$mathscr{I}_{spacetext{n}}left(text{m}space,spacetext{s}right)=text{n}cdotfrac{expleft(frac{text{s}^2}{4cdotpicdottext{n}^2}right)}{2cdottext{n}}cdotleft{text{erf}left(frac{2cdotpicdottext{n}^2cdottext{m}+text{s}}{2cdotsqrt{pi}cdottext{n}}right)-text{erf}left(frac{text{s}}{2cdotsqrt{pi}cdottext{n}}right)right}tag4$$
Now, your task is to simplify and to take:
$$lim_{text{m}toinfty}mathscr{I}_{spacetext{n}}left(text{m}space,spacetext{s}right)tag5$$
Try to prove:
$$lim_{text{m}toinfty}text{erf}left(frac{2cdotpicdottext{n}^2cdottext{m}+text{s}}{2cdotsqrt{pi}cdottext{n}}right)=1tag6$$
answered Sep 18 '17 at 15:55
JanJan
22.1k31340
22.1k31340
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$begingroup$
This is as far as it goes: the answer is in terms of the cumulative normal distribution function.
$endgroup$
– uniquesolution
Sep 18 '17 at 15:53
$begingroup$
finishing the square yields another gaussian
$endgroup$
– alain
Jan 22 '18 at 10:51