Help Solving Integration $int_-infty^infty (textStandard Normal times textSigmoid) $

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Does anyone know how to analytically solve the below expression?



$$int_-infty^infty p(x) times sigma(ax + b) , dx $$



where $ p(x) = N(0, 1) = frac1sqrt2pie^frac-x^22$ and $sigma(z) = frac11+e^-z$.



I know that the integration of standard Gaussian is $pi$. Moreover, the integration of sigmoid alone is,



$$int sigma(x) , df = int frace^x1+e^x , dx = x + c$$



But I cant figure out how to integrate the first equation), when they are multiplied together.



Can we use integration by part and solve above equation?



Any help will be appreciated.
Thanks



Edit 1:



Sorry about the complicated notations.
If we simplify the above expression, then we get following equation.
$$ frac1sqrt2pi int_-infty^infty frace^-fracx^221+e^-x dx $$







share|cite|improve this question





















  • What is $p(x)$ ? Is it the probability density function ?
    – MysteryGuy
    Aug 2 at 12:11










  • yes. $p(x) = frac1sqrt2pie^frac-x^22$
    – Nadheesh
    Aug 2 at 12:17











  • If $x$ is between $pminfty,$ as opposed to $x$ having an imaginary part, then $ln|e^x|$ is redundant and one can just write $ln e^x,$ and $ln e^x$ is the same as $x,$ so $ln e^x + c = x + c. qquad$
    – Michael Hardy
    Aug 2 at 12:26










  • @MichaelHardy thanks for the suggestion. edited the post accordingly. However, that is not what I'm interested in. I want to solve the first expression.
    – Nadheesh
    Aug 2 at 12:33










  • Sorry I’m not familiar with this topic...but what is $N(0,1)$?
    – Szeto
    Aug 2 at 12:35














up vote
2
down vote

favorite
3












Does anyone know how to analytically solve the below expression?



$$int_-infty^infty p(x) times sigma(ax + b) , dx $$



where $ p(x) = N(0, 1) = frac1sqrt2pie^frac-x^22$ and $sigma(z) = frac11+e^-z$.



I know that the integration of standard Gaussian is $pi$. Moreover, the integration of sigmoid alone is,



$$int sigma(x) , df = int frace^x1+e^x , dx = x + c$$



But I cant figure out how to integrate the first equation), when they are multiplied together.



Can we use integration by part and solve above equation?



Any help will be appreciated.
Thanks



Edit 1:



Sorry about the complicated notations.
If we simplify the above expression, then we get following equation.
$$ frac1sqrt2pi int_-infty^infty frace^-fracx^221+e^-x dx $$







share|cite|improve this question





















  • What is $p(x)$ ? Is it the probability density function ?
    – MysteryGuy
    Aug 2 at 12:11










  • yes. $p(x) = frac1sqrt2pie^frac-x^22$
    – Nadheesh
    Aug 2 at 12:17











  • If $x$ is between $pminfty,$ as opposed to $x$ having an imaginary part, then $ln|e^x|$ is redundant and one can just write $ln e^x,$ and $ln e^x$ is the same as $x,$ so $ln e^x + c = x + c. qquad$
    – Michael Hardy
    Aug 2 at 12:26










  • @MichaelHardy thanks for the suggestion. edited the post accordingly. However, that is not what I'm interested in. I want to solve the first expression.
    – Nadheesh
    Aug 2 at 12:33










  • Sorry I’m not familiar with this topic...but what is $N(0,1)$?
    – Szeto
    Aug 2 at 12:35












up vote
2
down vote

favorite
3









up vote
2
down vote

favorite
3






3





Does anyone know how to analytically solve the below expression?



$$int_-infty^infty p(x) times sigma(ax + b) , dx $$



where $ p(x) = N(0, 1) = frac1sqrt2pie^frac-x^22$ and $sigma(z) = frac11+e^-z$.



I know that the integration of standard Gaussian is $pi$. Moreover, the integration of sigmoid alone is,



$$int sigma(x) , df = int frace^x1+e^x , dx = x + c$$



But I cant figure out how to integrate the first equation), when they are multiplied together.



Can we use integration by part and solve above equation?



Any help will be appreciated.
Thanks



Edit 1:



Sorry about the complicated notations.
If we simplify the above expression, then we get following equation.
$$ frac1sqrt2pi int_-infty^infty frace^-fracx^221+e^-x dx $$







share|cite|improve this question













Does anyone know how to analytically solve the below expression?



$$int_-infty^infty p(x) times sigma(ax + b) , dx $$



where $ p(x) = N(0, 1) = frac1sqrt2pie^frac-x^22$ and $sigma(z) = frac11+e^-z$.



I know that the integration of standard Gaussian is $pi$. Moreover, the integration of sigmoid alone is,



$$int sigma(x) , df = int frace^x1+e^x , dx = x + c$$



But I cant figure out how to integrate the first equation), when they are multiplied together.



Can we use integration by part and solve above equation?



Any help will be appreciated.
Thanks



Edit 1:



Sorry about the complicated notations.
If we simplify the above expression, then we get following equation.
$$ frac1sqrt2pi int_-infty^infty frace^-fracx^221+e^-x dx $$









share|cite|improve this question












share|cite|improve this question




share|cite|improve this question








edited 21 hours ago
























asked Aug 2 at 12:06









Nadheesh

165




165











  • What is $p(x)$ ? Is it the probability density function ?
    – MysteryGuy
    Aug 2 at 12:11










  • yes. $p(x) = frac1sqrt2pie^frac-x^22$
    – Nadheesh
    Aug 2 at 12:17











  • If $x$ is between $pminfty,$ as opposed to $x$ having an imaginary part, then $ln|e^x|$ is redundant and one can just write $ln e^x,$ and $ln e^x$ is the same as $x,$ so $ln e^x + c = x + c. qquad$
    – Michael Hardy
    Aug 2 at 12:26










  • @MichaelHardy thanks for the suggestion. edited the post accordingly. However, that is not what I'm interested in. I want to solve the first expression.
    – Nadheesh
    Aug 2 at 12:33










  • Sorry I’m not familiar with this topic...but what is $N(0,1)$?
    – Szeto
    Aug 2 at 12:35
















  • What is $p(x)$ ? Is it the probability density function ?
    – MysteryGuy
    Aug 2 at 12:11










  • yes. $p(x) = frac1sqrt2pie^frac-x^22$
    – Nadheesh
    Aug 2 at 12:17











  • If $x$ is between $pminfty,$ as opposed to $x$ having an imaginary part, then $ln|e^x|$ is redundant and one can just write $ln e^x,$ and $ln e^x$ is the same as $x,$ so $ln e^x + c = x + c. qquad$
    – Michael Hardy
    Aug 2 at 12:26










  • @MichaelHardy thanks for the suggestion. edited the post accordingly. However, that is not what I'm interested in. I want to solve the first expression.
    – Nadheesh
    Aug 2 at 12:33










  • Sorry I’m not familiar with this topic...but what is $N(0,1)$?
    – Szeto
    Aug 2 at 12:35















What is $p(x)$ ? Is it the probability density function ?
– MysteryGuy
Aug 2 at 12:11




What is $p(x)$ ? Is it the probability density function ?
– MysteryGuy
Aug 2 at 12:11












yes. $p(x) = frac1sqrt2pie^frac-x^22$
– Nadheesh
Aug 2 at 12:17





yes. $p(x) = frac1sqrt2pie^frac-x^22$
– Nadheesh
Aug 2 at 12:17













If $x$ is between $pminfty,$ as opposed to $x$ having an imaginary part, then $ln|e^x|$ is redundant and one can just write $ln e^x,$ and $ln e^x$ is the same as $x,$ so $ln e^x + c = x + c. qquad$
– Michael Hardy
Aug 2 at 12:26




If $x$ is between $pminfty,$ as opposed to $x$ having an imaginary part, then $ln|e^x|$ is redundant and one can just write $ln e^x,$ and $ln e^x$ is the same as $x,$ so $ln e^x + c = x + c. qquad$
– Michael Hardy
Aug 2 at 12:26












@MichaelHardy thanks for the suggestion. edited the post accordingly. However, that is not what I'm interested in. I want to solve the first expression.
– Nadheesh
Aug 2 at 12:33




@MichaelHardy thanks for the suggestion. edited the post accordingly. However, that is not what I'm interested in. I want to solve the first expression.
– Nadheesh
Aug 2 at 12:33












Sorry I’m not familiar with this topic...but what is $N(0,1)$?
– Szeto
Aug 2 at 12:35




Sorry I’m not familiar with this topic...but what is $N(0,1)$?
– Szeto
Aug 2 at 12:35










2 Answers
2






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up vote
4
down vote



accepted










beginalign
frac1sqrt2pi int_-infty^infty e^-fracx^22frace^x1+e^x dx&=frac1sqrt2pi int_0^infty e^-fracx^22frace^x1+e^x dx+frac1sqrt2pi int_-infty^0 e^-fracx^22frace^x1+e^x dx\
&=frac1sqrt2pi int_0^infty e^-fracx^22frace^x1+e^x dx+frac1sqrt2pi int_0^infty e^-fracx^22frace^-x1+e^-x dx\
&=frac1sqrt2pi int_0^infty e^-fracx^22frace^x1+e^x dx+frac1sqrt2pi int_0^infty e^-fracx^22frac11+e^x dx\
&=frac1sqrt2pi int_0^infty e^-fracx^22 dx\
&=frac12
endalign






share|cite|improve this answer





















  • Thanks for all the work, I would have never thought of this myself. How can we solve the expression if the sigmoid function include some other parameters as show below? $$ frac1sqrt2pi int_-infty^infty frace^-fracx^221+e^-(ax+b) dx $$
    – Nadheesh
    Aug 3 at 1:54


















up vote
2
down vote













$$
varphi(x) = frac 1 sqrt2pi e^-x^2/2
$$
$$
sigma (x) = frac 1 1+e^-x
$$
Some simple algebra shows that
$$
sigma(-x) = 1 - sigma(x)
$$
and it is even easier to show that
$$
varphi(-x) = varphi(x),
$$
so we have
$$
int_-infty^0 varphi(x),dx = int_0^infty varphi(x),dx = frac 1 2.
$$



Therefore
beginalign
int_-infty^0 sigma(x) varphi(x) , dx & = int_0^+infty (1-sigma(x)) varphi(x), dx \[10pt]
& = int_0^+infty varphi(x),dx - int_0^+infty sigma(x)varphi(x),dx
endalign
Therefore
$$
int_-infty^+infty sigma(x)varphi(x),dx = int_0^+infty varphi(x), dx = frac 1 2.
$$






share|cite|improve this answer



















  • 1




    Thanks for the solution. This seems pretty simple yet, I could not have think of without your help. However, if we have some other variables within the sigmoid function, then how can we solve the same expression. I tried substituting z = ax+b, then I could not get rid of the sigmoid. $$ frac1sqrt2pi int_-infty^infty frace^-fracx^221+e^-(ax+b) dx $$
    – Nadheesh
    Aug 3 at 1:57











  • do you think if we can solve this expression when the sigmoid function is changed as shown above.
    – Nadheesh
    Aug 4 at 2:48










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2 Answers
2






active

oldest

votes








2 Answers
2






active

oldest

votes









active

oldest

votes






active

oldest

votes








up vote
4
down vote



accepted










beginalign
frac1sqrt2pi int_-infty^infty e^-fracx^22frace^x1+e^x dx&=frac1sqrt2pi int_0^infty e^-fracx^22frace^x1+e^x dx+frac1sqrt2pi int_-infty^0 e^-fracx^22frace^x1+e^x dx\
&=frac1sqrt2pi int_0^infty e^-fracx^22frace^x1+e^x dx+frac1sqrt2pi int_0^infty e^-fracx^22frace^-x1+e^-x dx\
&=frac1sqrt2pi int_0^infty e^-fracx^22frace^x1+e^x dx+frac1sqrt2pi int_0^infty e^-fracx^22frac11+e^x dx\
&=frac1sqrt2pi int_0^infty e^-fracx^22 dx\
&=frac12
endalign






share|cite|improve this answer





















  • Thanks for all the work, I would have never thought of this myself. How can we solve the expression if the sigmoid function include some other parameters as show below? $$ frac1sqrt2pi int_-infty^infty frace^-fracx^221+e^-(ax+b) dx $$
    – Nadheesh
    Aug 3 at 1:54















up vote
4
down vote



accepted










beginalign
frac1sqrt2pi int_-infty^infty e^-fracx^22frace^x1+e^x dx&=frac1sqrt2pi int_0^infty e^-fracx^22frace^x1+e^x dx+frac1sqrt2pi int_-infty^0 e^-fracx^22frace^x1+e^x dx\
&=frac1sqrt2pi int_0^infty e^-fracx^22frace^x1+e^x dx+frac1sqrt2pi int_0^infty e^-fracx^22frace^-x1+e^-x dx\
&=frac1sqrt2pi int_0^infty e^-fracx^22frace^x1+e^x dx+frac1sqrt2pi int_0^infty e^-fracx^22frac11+e^x dx\
&=frac1sqrt2pi int_0^infty e^-fracx^22 dx\
&=frac12
endalign






share|cite|improve this answer





















  • Thanks for all the work, I would have never thought of this myself. How can we solve the expression if the sigmoid function include some other parameters as show below? $$ frac1sqrt2pi int_-infty^infty frace^-fracx^221+e^-(ax+b) dx $$
    – Nadheesh
    Aug 3 at 1:54













up vote
4
down vote



accepted







up vote
4
down vote



accepted






beginalign
frac1sqrt2pi int_-infty^infty e^-fracx^22frace^x1+e^x dx&=frac1sqrt2pi int_0^infty e^-fracx^22frace^x1+e^x dx+frac1sqrt2pi int_-infty^0 e^-fracx^22frace^x1+e^x dx\
&=frac1sqrt2pi int_0^infty e^-fracx^22frace^x1+e^x dx+frac1sqrt2pi int_0^infty e^-fracx^22frace^-x1+e^-x dx\
&=frac1sqrt2pi int_0^infty e^-fracx^22frace^x1+e^x dx+frac1sqrt2pi int_0^infty e^-fracx^22frac11+e^x dx\
&=frac1sqrt2pi int_0^infty e^-fracx^22 dx\
&=frac12
endalign






share|cite|improve this answer













beginalign
frac1sqrt2pi int_-infty^infty e^-fracx^22frace^x1+e^x dx&=frac1sqrt2pi int_0^infty e^-fracx^22frace^x1+e^x dx+frac1sqrt2pi int_-infty^0 e^-fracx^22frace^x1+e^x dx\
&=frac1sqrt2pi int_0^infty e^-fracx^22frace^x1+e^x dx+frac1sqrt2pi int_0^infty e^-fracx^22frace^-x1+e^-x dx\
&=frac1sqrt2pi int_0^infty e^-fracx^22frace^x1+e^x dx+frac1sqrt2pi int_0^infty e^-fracx^22frac11+e^x dx\
&=frac1sqrt2pi int_0^infty e^-fracx^22 dx\
&=frac12
endalign







share|cite|improve this answer













share|cite|improve this answer



share|cite|improve this answer











answered Aug 2 at 13:35









Math-fun

6,8811225




6,8811225











  • Thanks for all the work, I would have never thought of this myself. How can we solve the expression if the sigmoid function include some other parameters as show below? $$ frac1sqrt2pi int_-infty^infty frace^-fracx^221+e^-(ax+b) dx $$
    – Nadheesh
    Aug 3 at 1:54

















  • Thanks for all the work, I would have never thought of this myself. How can we solve the expression if the sigmoid function include some other parameters as show below? $$ frac1sqrt2pi int_-infty^infty frace^-fracx^221+e^-(ax+b) dx $$
    – Nadheesh
    Aug 3 at 1:54
















Thanks for all the work, I would have never thought of this myself. How can we solve the expression if the sigmoid function include some other parameters as show below? $$ frac1sqrt2pi int_-infty^infty frace^-fracx^221+e^-(ax+b) dx $$
– Nadheesh
Aug 3 at 1:54





Thanks for all the work, I would have never thought of this myself. How can we solve the expression if the sigmoid function include some other parameters as show below? $$ frac1sqrt2pi int_-infty^infty frace^-fracx^221+e^-(ax+b) dx $$
– Nadheesh
Aug 3 at 1:54











up vote
2
down vote













$$
varphi(x) = frac 1 sqrt2pi e^-x^2/2
$$
$$
sigma (x) = frac 1 1+e^-x
$$
Some simple algebra shows that
$$
sigma(-x) = 1 - sigma(x)
$$
and it is even easier to show that
$$
varphi(-x) = varphi(x),
$$
so we have
$$
int_-infty^0 varphi(x),dx = int_0^infty varphi(x),dx = frac 1 2.
$$



Therefore
beginalign
int_-infty^0 sigma(x) varphi(x) , dx & = int_0^+infty (1-sigma(x)) varphi(x), dx \[10pt]
& = int_0^+infty varphi(x),dx - int_0^+infty sigma(x)varphi(x),dx
endalign
Therefore
$$
int_-infty^+infty sigma(x)varphi(x),dx = int_0^+infty varphi(x), dx = frac 1 2.
$$






share|cite|improve this answer



















  • 1




    Thanks for the solution. This seems pretty simple yet, I could not have think of without your help. However, if we have some other variables within the sigmoid function, then how can we solve the same expression. I tried substituting z = ax+b, then I could not get rid of the sigmoid. $$ frac1sqrt2pi int_-infty^infty frace^-fracx^221+e^-(ax+b) dx $$
    – Nadheesh
    Aug 3 at 1:57











  • do you think if we can solve this expression when the sigmoid function is changed as shown above.
    – Nadheesh
    Aug 4 at 2:48














up vote
2
down vote













$$
varphi(x) = frac 1 sqrt2pi e^-x^2/2
$$
$$
sigma (x) = frac 1 1+e^-x
$$
Some simple algebra shows that
$$
sigma(-x) = 1 - sigma(x)
$$
and it is even easier to show that
$$
varphi(-x) = varphi(x),
$$
so we have
$$
int_-infty^0 varphi(x),dx = int_0^infty varphi(x),dx = frac 1 2.
$$



Therefore
beginalign
int_-infty^0 sigma(x) varphi(x) , dx & = int_0^+infty (1-sigma(x)) varphi(x), dx \[10pt]
& = int_0^+infty varphi(x),dx - int_0^+infty sigma(x)varphi(x),dx
endalign
Therefore
$$
int_-infty^+infty sigma(x)varphi(x),dx = int_0^+infty varphi(x), dx = frac 1 2.
$$






share|cite|improve this answer



















  • 1




    Thanks for the solution. This seems pretty simple yet, I could not have think of without your help. However, if we have some other variables within the sigmoid function, then how can we solve the same expression. I tried substituting z = ax+b, then I could not get rid of the sigmoid. $$ frac1sqrt2pi int_-infty^infty frace^-fracx^221+e^-(ax+b) dx $$
    – Nadheesh
    Aug 3 at 1:57











  • do you think if we can solve this expression when the sigmoid function is changed as shown above.
    – Nadheesh
    Aug 4 at 2:48












up vote
2
down vote










up vote
2
down vote









$$
varphi(x) = frac 1 sqrt2pi e^-x^2/2
$$
$$
sigma (x) = frac 1 1+e^-x
$$
Some simple algebra shows that
$$
sigma(-x) = 1 - sigma(x)
$$
and it is even easier to show that
$$
varphi(-x) = varphi(x),
$$
so we have
$$
int_-infty^0 varphi(x),dx = int_0^infty varphi(x),dx = frac 1 2.
$$



Therefore
beginalign
int_-infty^0 sigma(x) varphi(x) , dx & = int_0^+infty (1-sigma(x)) varphi(x), dx \[10pt]
& = int_0^+infty varphi(x),dx - int_0^+infty sigma(x)varphi(x),dx
endalign
Therefore
$$
int_-infty^+infty sigma(x)varphi(x),dx = int_0^+infty varphi(x), dx = frac 1 2.
$$






share|cite|improve this answer















$$
varphi(x) = frac 1 sqrt2pi e^-x^2/2
$$
$$
sigma (x) = frac 1 1+e^-x
$$
Some simple algebra shows that
$$
sigma(-x) = 1 - sigma(x)
$$
and it is even easier to show that
$$
varphi(-x) = varphi(x),
$$
so we have
$$
int_-infty^0 varphi(x),dx = int_0^infty varphi(x),dx = frac 1 2.
$$



Therefore
beginalign
int_-infty^0 sigma(x) varphi(x) , dx & = int_0^+infty (1-sigma(x)) varphi(x), dx \[10pt]
& = int_0^+infty varphi(x),dx - int_0^+infty sigma(x)varphi(x),dx
endalign
Therefore
$$
int_-infty^+infty sigma(x)varphi(x),dx = int_0^+infty varphi(x), dx = frac 1 2.
$$







share|cite|improve this answer















share|cite|improve this answer



share|cite|improve this answer








edited 21 hours ago









Jean-Claude Arbaut

14.1k63260




14.1k63260











answered Aug 2 at 22:25









Michael Hardy

204k23185460




204k23185460







  • 1




    Thanks for the solution. This seems pretty simple yet, I could not have think of without your help. However, if we have some other variables within the sigmoid function, then how can we solve the same expression. I tried substituting z = ax+b, then I could not get rid of the sigmoid. $$ frac1sqrt2pi int_-infty^infty frace^-fracx^221+e^-(ax+b) dx $$
    – Nadheesh
    Aug 3 at 1:57











  • do you think if we can solve this expression when the sigmoid function is changed as shown above.
    – Nadheesh
    Aug 4 at 2:48












  • 1




    Thanks for the solution. This seems pretty simple yet, I could not have think of without your help. However, if we have some other variables within the sigmoid function, then how can we solve the same expression. I tried substituting z = ax+b, then I could not get rid of the sigmoid. $$ frac1sqrt2pi int_-infty^infty frace^-fracx^221+e^-(ax+b) dx $$
    – Nadheesh
    Aug 3 at 1:57











  • do you think if we can solve this expression when the sigmoid function is changed as shown above.
    – Nadheesh
    Aug 4 at 2:48







1




1




Thanks for the solution. This seems pretty simple yet, I could not have think of without your help. However, if we have some other variables within the sigmoid function, then how can we solve the same expression. I tried substituting z = ax+b, then I could not get rid of the sigmoid. $$ frac1sqrt2pi int_-infty^infty frace^-fracx^221+e^-(ax+b) dx $$
– Nadheesh
Aug 3 at 1:57





Thanks for the solution. This seems pretty simple yet, I could not have think of without your help. However, if we have some other variables within the sigmoid function, then how can we solve the same expression. I tried substituting z = ax+b, then I could not get rid of the sigmoid. $$ frac1sqrt2pi int_-infty^infty frace^-fracx^221+e^-(ax+b) dx $$
– Nadheesh
Aug 3 at 1:57













do you think if we can solve this expression when the sigmoid function is changed as shown above.
– Nadheesh
Aug 4 at 2:48




do you think if we can solve this expression when the sigmoid function is changed as shown above.
– Nadheesh
Aug 4 at 2:48












 

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