Dirac Delta Function: Integral at x=a

In summary, the attempt at a solution states that it is not always the case that f(a)=-f(a) because at x=a, d(0)=infinite and integrating dirac delta over a region including x=0 when d(0) is in the value in the integral will produce 1 hence f(a).Think about it this way.
  • #1
pivoxa15
2,255
1

Homework Statement


int[d(x-a)f(x)dx]=f(a) is the dirac delta fn

but is int[d(a-x)f(x)]=f(a) as well? If so why?

The Attempt at a Solution


Is it because at x=a, d(0)=infinite and integrate dirac delta over a region including x=0 when d(0) is in the value in the integral will produce 1 hence f(a).
 
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  • #2
Think about it this way.

Since
[tex]\int f(x) d(a-x) = -\int f(x) d(x-a) = -f(a)[/tex] your asking is f(a)=-f(a).
 
  • #3
pivoxa15 said:

Homework Statement


int[d(x-a)f(x)dx]=f(a) is the dirac delta fn

but is int[d(a-x)f(x)]=f(a) as well? If so why?
No, [itex]\int_{-\infty}^{\infty} \delta (a- x)f(x)dx= -f(a)[/itex]. Use the substitution u= a-x.


The Attempt at a Solution


Is it because at x=a, d(0)=infinite and integrate dirac delta over a region including x=0 when d(0) is in the value in the integral will produce 1 hence f(a).
That's a very rough way of putting it. More correctly the dirac delta function (actually a "generalized function" or "distribution") is DEFINED by the property that [itex]\int \delta(x) f(x)dx = f(0)[/itex] as long as the integration include x= 0.
 
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  • #4
So it can be but in some cases it is not.
 
  • #5
Yup, choose your a.
 
  • #6
is it possible to explain step by step including the integration of the following:

(1) f(x) d(x-a) = f(a) d(x-a)

(2) \int f(a) d(a-x) = f(x)

Thank you,..!
 
  • #7
Are you changing symbols here? does "d(x-a)" still mean the delta function? Are you asking why
(1) [tex]\int f(x)\delta(x- a)dx= \int f(a)\delta(x-a)dx[/tex]?

That is because the definition of [itex]\delta(x-a)[/itex] require that the
[tex]\int f(x)\delta(x-a)dx= f(a)[/tex]
for any function f as long as the integral of integration includes x= a.
Of course,
[tex]\int f(a)\delta(x-a) dx= f(a)\int \delta(x-a)dx= f(a)(1)= f(a)[/tex]

As for
(2)[tex]\int f(a)d(a- x) dx= f(x)[/tex]
That's not true. In fact,the right hand side cannot be a function of x at all. The integral is just as in (1).
 

Related to Dirac Delta Function: Integral at x=a

What is the Dirac Delta Function?

The Dirac Delta Function is a mathematical function that is defined as zero everywhere except at a single point, where it is infinite. It is commonly represented by the symbol δ(x) or δa(x), where a is the point where the function is non-zero.

How is the Dirac Delta Function used in physics?

The Dirac Delta Function is often used in physics to model point masses or point charges. It is also used to represent impulsive forces or impulses in physical systems.

What is the integral of the Dirac Delta Function at a specific point?

The integral of the Dirac Delta Function at a point a is equal to 1. This is because the function is defined as zero everywhere except at the point a, where it is infinite.

Can the Dirac Delta Function be graphed?

No, the Dirac Delta Function cannot be graphed in the traditional sense because it is not a continuous function. However, it can be represented using a spike or impulse at the point a.

What is the relationship between the Dirac Delta Function and the Kronecker Delta Function?

The Kronecker Delta Function is a discrete version of the Dirac Delta Function. It is defined as 1 at the point a and 0 at all other points. In the limit as the spacing between the points approaches zero, the Kronecker Delta Function becomes the Dirac Delta Function.

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