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MathGroup Archive 2007

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Re: Problem when NIntegrate matrix

  • To: mathgroup at smc.vnet.net
  • Subject: [mg81360] Re: Problem when NIntegrate matrix
  • From: Szabolcs Horvát <szhorvat at gmail.com>
  • Date: Thu, 20 Sep 2007 03:54:28 -0400 (EDT)
  • References: <fcqrrj$2jc$1@smc.vnet.net>

chiron1201 wrote:
> Hi all!
> 
> I can not understand why there is a NIntegrate::inum error when I try to evaluate the following code
> 
> TEST = Piecewise[{
>      {{{1, 2, 3}, {2, 3, 4}, {3, 4, 5}}, #1 < 0.00},
>      {{{2, 3, 4}, {3, 4, 5}, {4, 5, 6}}, #1 >= 0.00},
>      }] &;
> 
> NIntegrate[TEST[x], {x, -1, +1}]

My guess is that NIntegrate[] evaluates its argument without x having a 
numerical value (perhaps it tries to Compile[] it?), and makes 
assumptions about the dimensions of the integrand based on this result.

Piecewise[...] is not a matrix (List of Lists), but a single expression. 
  But with a numerical argument is evaluates to a matrix.  This might 
confuse NIntegrate[].


In[1]:=
f[x_?NumericQ] := {1, 2}
g[x_] := {1, 2}
h[x_] := {x, 2 x}

In[4]:= NIntegrate[f[x], {x, 0, 3}]

During evaluation of In[4]:= NIntegrate::inum: Integrand f[x] is not \
numerical at {x} = {0.023872}. >>

During evaluation of In[4]:= NIntegrate::inum: Integrand f[x] is not \
numerical at {x} = {0.023872}. >>

Out[4]= NIntegrate[f[x], {x, 0, 3}]

In[5]:= NIntegrate[g[x], {x, 0, 3}]
Out[5]= {3., 6.}

In[6]:= NIntegrate[h[x], {x, 0, 3}]
Out[6]= {4.5, 9.}


Of course, this explanation does not solve your problem ... (and it 
might even be flawed) ...

-- 
Szabolcs


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