Re: slow integration

• To: mathgroup at smc.vnet.net
• Subject: [mg30050] Re: slow integration
• From: "Joshua A. Solomon" <J.A.Solomon at city.ac.uk>
• Date: Wed, 25 Jul 2001 01:00:34 -0400 (EDT)
• Organization: Posted via ULCC Internet Services
• References: <200107180608.CAA18960@smc.vnet.net> <9j65j1\$2gh\$1@smc.vnet.net> <200107200728.DAA05639@smc.vnet.net> <9jisf9\$eu\$1@smc.vnet.net>
• Sender: owner-wri-mathgroup at wolfram.com

```Ah, semantic confusion. Note that

In[1]:=
Timing[InputForm[Integrate[Erf[400 (z/x-.002)],{x,0,.03}]]]

produces an Out[1], but

In[2]:=
Integrate[E^(-100000 (.03-x)^2)Erf[400 (z/x-.002)],{x,0,.03}]

doesn't produce anything. Mathematica just thinks and thinks and thinks.

js
PS Tomas, my email to you seems to have been delayed.

On 24/7/01 5:10 am, in article 9jisf9\$eu\$1 at smc.vnet.net, "Tomas Garza"
<tgarza01 at prodigy.net.mx> wrote:

> You say "the integral is evaluated without delay":
>
> In[1]:=
>> Timing[InputForm[Integrate[Erf[400 (z/x-.002)],{x,0,.03}]]]
>> Out[1]=
>> {0.45 Second,Integrate[Erf[400*(-0.002 + z/x)], {x, 0, 0.03}]}
>
> But, is this an evaluation or just writing back what you gave as input?
>
> Tomas Garza
> Mexico City
>
> ----- Original Message -----
> From: "Joshua A. Solomon" <J.A.Solomon at city.ac.uk>
To: mathgroup at smc.vnet.net
> Subject: [mg30050]  Re: slow integration
>
>
>> I'm not sure that you really understand the problem. (I *am* sure that I
> do
>> not understand the problem.) Note that, without the Gaussian factor in the
>> integrand, the integral is evaluated without delay:
>>
>> In[1]:=
>> Timing[InputForm[Integrate[Erf[400 (z/x-.002)],{x,0,.03}]]]
>> Out[1]=
>> {0.45 Second,Integrate[Erf[400*(-0.002 + z/x)], {x, 0, 0.03}]}
>>
>>
>> On 19/7/01 9:26 am, in article 9j65j1\$2gh\$1 at smc.vnet.net, "Tomas Garza"
>> <tgarza01 at prodigy.net.mx> wrote:
>>
>>> You will not get any result. I guess the problem is that Erf[z] doesn't
> have
>>> a closed form and it remains expressed as a definite integral.
>>>
>>> Tomas Garza
>>> Mexico City
>>>
>>>
>>> ----- Original Message -----
>>> From: "Joshua A. Solomon" <J.A.Solomon at city.ac.uk>
To: mathgroup at smc.vnet.net
>> To: mathgroup at smc.vnet.net
>>> Subject: [mg30050]   slow integration
>>>
>>>
>>>> In[1]:=
>>>> \$Version
>>>> Out[1]=
>>>> 4.1 for Power Macintosh (November 2, 2000)
>>>>
>>>> Does anyone out there understand why, after two hours, the following
> still
>>>> hasn't produced any output?
>>>>
>>>> In[2]:=
>>>> Integrate[E^(-100000 (.03-x)^2)Erf[400 (z/x-.002)],{x,0,.03}]
>>>>
>>>> js
>>>> --
>>>> Joshua A. Solomon
>>>> Department of Optometry and Visual Science
>>>> City University
>>>> London EC1V 0HB
>>>> Voice: (44) 20 7040 0192
>>>> Secretary/Fax: (44) 20 7040 0182
>>>> J.A.Solomon at city.ac.uk
>>>> http://www.staff.city.ac.uk/~solomon
>>>>
>>>>
>>>
>>>
>>
>>
>
>

```

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