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Forum index » Science and Technology » Math » num-analysis
Integrate erf type expression
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Pratim Vakish
science forum beginner


Joined: 17 Jul 2006
Posts: 2

PostPosted: Mon Jul 17, 2006 2:27 pm    Post subject: Integrate erf type expression Reply with quote
My question is probably very basic. I am a novice in the field.

I would like to compute/simplify the following expression:

Integrale [from -infinity to z] of [e^-(x-y)^2]dx

Could you please help?

Best
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Peter Spellucci
science forum Guru


Joined: 29 Apr 2005
Posts: 702

PostPosted: Mon Jul 17, 2006 3:37 pm    Post subject: Re: Integrate erf type expression Reply with quote
In article <13642928.1153146486183.JavaMail.jakarta@nitrogen.mathforum.org>,
Pratim Vakish <pratim_usc@hotmail.com> writes:
Quote:
My question is probably very basic. I am a novice in the field.

I would like to compute/simplify the following expression:

Integrale [from -infinity to z] of [e^-(x-y)^2]dx

Could you please help?

Best

what about a little substitution to get it into
const* integral_{-infinity to somewhere} exp(-t^2/2) dt
then using the normal distribution function?
hth
peter
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Badger
science forum beginner


Joined: 07 May 2006
Posts: 38

PostPosted: Mon Jul 17, 2006 4:16 pm    Post subject: Re: Integrate erf type expression Reply with quote
On Mon, 17 Jul 2006 10:27:35 EDT, Pratim Vakish
<pratim_usc@hotmail.com> wrote:

Quote:
My question is probably very basic. I am a novice in the field.

I would like to compute/simplify the following expression:

Integrale [from -infinity to z] of [e^-(x-y)^2]dx

Could you please help?

Best

In what follows, I have used Mathematica's definitions of Erf and Erfc
as given here:
<http://mathworld.wolfram.com/Erf.html>
<http://mathworld.wolfram.com/Erfc.html>

Some authors define these functions without the factor 2/sqrt(pi).

Given
I = Int[-oo...z] e^-(x-y)^2 dx

= sqrt(pi)/2 Erf(x-y) | [-oo...z]

= sqrt(pi)/2 [ Erf(z-y) - Erf(-oo-y) ]

Now, Erf is an odd function and Erf(oo) = 1, so

I = sqrt(pi)/2 [ -Erf(y-z) + Erf(oo) ]

= sqrt(pi)/2 [ 1 - Erf(y-z) ]

I = sqrt(pi)/2 Erfc(y-z)


The 'erf' functions can be evaluated numerically using Mathematica
here:
<http://functions.wolfram.com/GammaBetaErf/>

Click on Erfc, then 'Evaluation'.

HTH
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Pratim Vakish
science forum beginner


Joined: 17 Jul 2006
Posts: 2

PostPosted: Mon Jul 17, 2006 10:20 pm    Post subject: Re: Integrate erf type expression Reply with quote
Thank you very much for your answer Badger.

Just an additional question to be sure I have fully understood.

Let me consider this expression:

I = Int[-oo...z] e^-1/2*((x-y)/0.1)^2 dx

= sqrt(pi)/2 *
Erf(x/(sqrt(2)*0.1) - y/((sqrt(2)*0.1))) | [-oo...z]

= sqrt(pi)/2 [Erf(z/(sqrt(2)*0.1) - y/(sqrt(2)*0.1))
- Erf(-oo-y/(sqrt(2)*0.1)) ]

Now, Erf is an odd function and Erf(oo) = 1, so

I = sqrt(pi)/2 [ -Erf(y / (sqrt(2)*0.1) - z (sqrt(2)*0.1)) + Erf(oo) ]

= sqrt(pi)/2 [ 1 - Erf(y/(sqrt(2)*0.1)-z/(sqrt(2)*0.1))]

Now if I take the derivative of the expression above with respect to y, is it correct to say that it is equal to:

= - sqrt(pi)/2*2/sqrt(pi) * 1/(sqrt(2)*0.1) *
e^[(y/(sqrt(2)*0.1)-z/(sqrt(2)*0.1))]

= -1/(sqrt(2)*0.1) *
e^[(y/(sqrt(2)*0.1)-z/(sqrt(2)*0.1))]

Is it correct?
Is ther simplication I can do beforehand?
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Badger
science forum beginner


Joined: 07 May 2006
Posts: 38

PostPosted: Tue Jul 18, 2006 1:49 pm    Post subject: Re: Integrate erf type expression Reply with quote
On Mon, 17 Jul 2006 18:20:54 EDT, Pratim Vakish
<pratim_usc@hotmail.com> wrote:

Quote:
Thank you very much for your answer Badger.

Just an additional question to be sure I have fully understood.

Let me consider this expression:

I = Int[-oo...z] e^-1/2*((x-y)/0.1)^2 dx

= sqrt(pi)/2 *
Erf(x/(sqrt(2)*0.1) - y/((sqrt(2)*0.1))) | [-oo...z]


Not correct. Because of the constants you introduced into the
problem, there will be a new constant factor in the antiderivative.

To see this, let's say you want to evaluate

Int e^-( a (x-y) )^2 dx

where a is a real constant > 0 (this form is sufficient to do your new
integral).

Make the substitution

t = a (x-y)

Then dt = a dx, or dx = dt / a

The integral becomes

Int (1/a) e^-t^2 dt = sqrt(pi)/2 (1/a) Erf(t)

Notice the constant factor 1/a, and remember that the sqrt(pi)/2 is
present because of the way Mathematica defines the Erf function.

Now, substitute for t, and we have the antiderivative

sqrt(pi)/2 (1/a) Erf( a (x-y) )


For your new problem, you want to evaluate

I = Int[-oo...z] e^-1/2*((x-y)/0.1)^2 dx

which, if I'm reading it correctly, can be written

I = Int[-oo...z] e^-( (x-y) / (0.1 sqrt(2)) )^2 dx

= Int[-oo...z] e^-( 5 sqrt(2) (x-y) )^2 dx


So, using our result above, with a = 5 sqrt(2), we have

I = sqrt(pi)/2 1/(5 sqrt(2)) Erf( 5 sqrt(2) (x-y) ) | [-oo...z]

= sqrt(pi)/2 1/(5 sqrt(2)) [ Erf( 5 sqrt(2) (z-y) ) - Erf(-oo) ]

= sqrt(pi)/2 1/(5 sqrt(2)) [ -Erf( 5 sqrt(2) (y-z) ) + 1 ]

= sqrt(pi)/2 1/(5 sqrt(2)) [ 1 - Erf( 5 sqrt(2) (y-z) ) ]

I = sqrt(pi)/2 1/(5 sqrt(2)) Erfc( 5 sqrt(2) (y-z) )

Quote:

= sqrt(pi)/2 [Erf(z/(sqrt(2)*0.1) - y/(sqrt(2)*0.1))
- Erf(-oo-y/(sqrt(2)*0.1)) ]

Now, Erf is an odd function and Erf(oo) = 1, so

I = sqrt(pi)/2 [ -Erf(y / (sqrt(2)*0.1) - z (sqrt(2)*0.1)) + Erf(oo) ]

= sqrt(pi)/2 [ 1 - Erf(y/(sqrt(2)*0.1)-z/(sqrt(2)*0.1))]

Now if I take the derivative of the expression above with respect to y, is it correct to say that it is equal to:

= - sqrt(pi)/2*2/sqrt(pi) * 1/(sqrt(2)*0.1) *
e^[(y/(sqrt(2)*0.1)-z/(sqrt(2)*0.1))]

= -1/(sqrt(2)*0.1) *
e^[(y/(sqrt(2)*0.1)-z/(sqrt(2)*0.1))]

Is it correct?
Is ther simplication I can do beforehand?

Not correct. You say that you want the derivative with respect to y?
Why not with respect to z? I'll do it with respect to z.


I' = d/dz sqrt(pi)/2 1/(5 sqrt(2)) Erfc( 5 sqrt(2) (y-z) )

First, I have it that

d/dz Erfc( 5 sqrt(2) (y-z) )

= 2/sqrt(pi) [ e^-( 5 sqrt(2) (y-z) )^2 ] (5 sqrt(2))

and so

I' = e^-( 5 sqrt(2) (y-z) )^2

which is equivalent to the original integrand.


Now, you should do all the work yourself and see if you agree with
these results.
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