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I try to evaluate an Integral of a Heaviside function, but it turns out with an erroneous output(zero). Here is my integral:

Integrate[HeavisideTheta[k - Sqrt[kx^2 + ky^2 + kz^2]], 
          {kx, -Infinity, Infinity}, 
          {ky, -Infinity, Infinity}, 
          {kz, -Infinity, Infinity}, {kx, ky, kz} ∈ Reals]

The right answer should be a volume of a sphere, but it seems Mathematica doesn't know it.

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Define k first. –  Spawn1701D May 13 '13 at 18:21
    
How to define k? Can you make it clear? I add Assumptions->k>0 in the integral. It returns 0. –  luming May 13 '13 at 19:13

1 Answer 1

up vote 3 down vote accepted

You can use :

Integrate[UnitStep[k - Sqrt[kx^2 + ky^2 + kz^2]], 
{kx, -Infinity, Infinity}, {ky, -Infinity, Infinity}, {kz, -Infinity, Infinity}, 
  Assumptions -> {k \[Element] Reals}]
(* -(4/3) k^3 \[Pi] (-1 + UnitStep[-k]) *)

Integrate[Boole[k - Sqrt[kx^2 + ky^2 + kz^2] >= 0],
{kx, -Infinity, Infinity}, {ky, -Infinity, Infinity}, {kz, -Infinity, Infinity}, 
 Assumptions -> {k \[Element] Reals}]
(* Piecewise[{{(4*k^3*Pi)/3, k >= 0}}, 0] *)
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UnitStep doesn't work. It result 0 as same as Heaviside Step. I need to Integrate a lot of HeavisideStep functions, What can I do? –  luming May 13 '13 at 19:04
    
@luming Why doesn't it work ? I think it's giving the correct result; if you supplement the more stringent assumption k>0 then you get the volume of the sphere as you expect. –  b.gatessucks May 13 '13 at 20:05
    
In version 8, both methods work even if I leave out the Assumptions. –  Jens May 14 '13 at 3:36
    
@Jens I try the code on two differnt computers, both run mathematica7 for students on Windows7. And one of the computers running mathematica8 on linux Fedora16. All of them give the answer 0 when using`UnitStep`and`HevisideTheta` function with or without Assumptions. –  luming May 14 '13 at 5:19
    
I can confirm that. In version 7 only the 1 dimensional case Integrate[UnitStep[k - Sqrt[kx^2]], {kx, -Infinity, Infinity}] seems to work. –  b.gatessucks May 14 '13 at 5:30

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