RE: Incorrect integral

• To: mathgroup at smc.vnet.net
• Subject: [mg43640] RE: [mg43623] Incorrect integral
• From: "Rasmusson, Lars" <larsr at exch.hpl.hp.com>
• Date: Sat, 27 Sep 2003 04:58:01 -0400 (EDT)
• Sender: owner-wri-mathgroup at wolfram.com

```Thank you for help.  It seems to work.

However, Mathematica does not seem to be working correctly when the integration boundaries are given symbolicly.  Is there a way to handle that as well?

To see that it is incorrect, compare the result of

In[1]:=
InputForm[f[p_] = Integrate[UnitStep[k-a*p]*(k-a*p),
{k,K0,K1}, {a,A0,A1}, Assumptions->0<p<1]]  ;

In[2]:=
f[0.1]/.{K0|A0->0,K1|A1->1}

Out[2]=
0.453333

with the correct value 0.451667.

Again, I'm very grateful for your good response to my question.

Best regards,
Lars Rasmusson

-----Original Message-----
From: Daniel Lichtblau [mailto:danl at wolfram.com]
To: mathgroup at smc.vnet.net
Subject: [mg43640] Re: [mg43623] Incorrect integral

Lars Rasmusson wrote:
>
> Hi, it seems like Integrate is not handling the Max[ ] function
> properly, or am I mistaken? Compare the two outputs
> In[1]:=
>
> fa[p_] := NIntegrate[Max[0, k - a p], {k, 0, 1}, {a, 0, 1}] fb[p_] :=
> Integrate[Max[0, k - a p], {k, 0, 1}, {a, 0, 1}] fa[0.1]
> fb[0.1]
>
> Out[3]=
> 0.451667
> Out[4]=
> 1.66667
>
> The outputs differ.  NIntegrate returns the correct value.
>
> In[5]:= \$Version
> Out[5]= 5.0 for Microsoft Windows (June 11, 2003)
>
> Is there any way to get a correct behavior from Mathematica?  I would
> be happy to receive the symbolic solution (to a more complicated
> integral of this kind).
>
> Thanks,
>
> Lars

One method that Integrate can handle is to reformulate using UnitStep instead of Max.

In[9]:= InputForm[f[p_] = Integrate[UnitStep[k-a*p]*(k-a*p),
{k,0,1}, {a,0,1}, Assumptions->0<p<1]]
Out[9]//InputForm= (3 - 3*p + p^2)/6

In[11]:= f[.1]
Out[11]= 0.451667

Daniel Lichtblau
Wolfram Research

```

• Prev by Date: Re: NDSolve help
• Next by Date: Re: guidance in mathematica programming in pde
• Previous by thread: Re: Incorrect integral
• Next by thread: Re: Incorrect integral