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MathGroup Archive 2005

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Re: a question about the UnitStep function

  • To: mathgroup at smc.vnet.net
  • Subject: [mg58424] Re: [mg58411] a question about the UnitStep function
  • From: Bob Hanlon <hanlonr at cox.net>
  • Date: Sat, 2 Jul 2005 04:06:22 -0400 (EDT)
  • Reply-to: hanlonr at cox.net
  • Sender: owner-wri-mathgroup at wolfram.com

f[x_]:=(UnitStep[x+1]-UnitStep[x-1])/2;

integrand=f[z] f[x-z];

A workaround is to use finite but "large" limits on the integral

g[x_]=Assuming[Element[x,Reals],
    Integrate[integrand,{z,-10,10}]]

(1/4)*((-(x + 2))*UnitStep[-x - 2] - (x - 2)*UnitStep[2 - x] + 2*x*UnitStep[-x])

g[x]//Simplify

Piecewise[{{(2 - x)/4, Inequality[0, Less, x, LessEqual, 2]}, {(x + 2)/4, 
Inequality[-2, Less, x, LessEqual, 0]}}]

Plot[g[x],{x,-4,4},PlotRange->All];


Bob Hanlon

> 
> From: Zhou Jiang <jiangzhou_yz at yahoo.com>
To: mathgroup at smc.vnet.net
> Date: 2005/07/01 Fri AM 02:02:03 EDT
> Subject: [mg58424] [mg58411] a question about the UnitStep function
> 
> 
> Dear Mathgroup, 
> I want to let Mathematica compute the convolution of two sqare waves. I 
did as follows
>  
> f[x_]:=(UnitStep[x+1]-UnitStep[x-1])/2;
>  
> integrand=f[z] f[x-z];
>  
> Assuming[Element[x, Reals], Integrate[integrand, {z, -Infinity, Infinity}]]
>  
> Mathematica gave me the result as follows,
> ((-1 + x) UnitStep[-1 + x] - x UnitStep[x] + (2 + x) UnitStep[2 + x])/4
>  
> I plot the result to check 
>  
> Plot[%,{x,-10,10}, PlotRange->All];
>  
> It is clear wrong since the convolution of two square waves should be 
convergent. Can anyone give me some help with the subtlties about the 
UnitStep function? Any thoughts are appriciable. 
>  
> 
> 


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