[Mathematica] Bug in Integrate with derivatives of a delta function

In summary, the issue with Integrate in Mathematica is that it does not always handle derivatives of a delta function correctly. Mathematica represents the delta function as DiracDelta[x] and its derivatives as Derivative[n][DiracDelta[x]], but the behavior of these derivatives can be inconsistent and may not match with the expected mathematical properties. There are some workarounds for dealing with this issue, such as using the HeavisideTheta function or manually defining the properties of the delta function. Wolfram has acknowledged this issue and is working on improving it in future versions. The issue has been reported in multiple versions of Mathematica, but the severity and frequency may vary.
  • #1
Hepth
Gold Member
464
40
Code:
Integrate[f[qs] DiracDelta'[qs (1 - 1/x)], {qs, -\[Infinity], \[Infinity]}, 
 Assumptions -> 0 < x < 1]
Integrate[f[qs] DiracDelta'[qs - qs/x], {qs, -\[Infinity], \[Infinity]}, 
 Assumptions -> 0 < x < 1]

This is on Mathematica 8 for windows.
The results differ by a sign. They are effective the exact same thing, but due to how I write the insides of the dirac delta I get a different result.

Can anyone replicate this?
 
Last edited:
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  • #2
I notice it is only for the first negative power of x. If you put

[qs (1 - 1/x^(1.))]

[qs - qs/x^(1.)]

it works?
 

1. What is the issue with Integrate and derivatives of a delta function in Mathematica?

The issue with Integrate in Mathematica is that it does not always handle derivatives of a delta function correctly. In certain cases, the output may be incorrect or the computation may not be possible at all.

2. How does Mathematica handle derivatives of a delta function?

In Mathematica, the delta function is represented as DiracDelta[x], and its derivatives are represented as Derivative[n][DiracDelta[x]]. However, the behavior of these derivatives can be inconsistent and may not match with the expected mathematical properties of the delta function.

3. Are there any workarounds for dealing with this issue in Mathematica?

Yes, there are some workarounds that can be used to handle derivatives of a delta function in Mathematica. One approach is to use the HeavisideTheta function instead of the delta function, as it has similar properties and is better supported in Mathematica. Another approach is to manually define the properties of the delta function and its derivatives using user-defined functions.

4. Is there any official statement from Wolfram on this issue?

Yes, Wolfram has acknowledged this issue and has stated that it is a known limitation in the current version of Mathematica. They have also provided some suggestions for workarounds and have stated that they are working on improving the handling of derivatives of a delta function in future versions.

5. Does this issue affect all versions of Mathematica?

Yes, this issue has been reported in multiple versions of Mathematica, including the latest version. However, the severity and frequency of the issue may vary depending on the specific version and the specific use case.

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