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Re: IDL is not accurate enough! [message #62508 is a reply to message #62192]

Thu, 11 September 2008 07:44

pgrigis
Messages: 436
Registered: September 2007

Senior Member

pgri...@gmail.com wrote:
> noahh.schwa...@gmail.com wrote:
>> On 28 ao�t, 18:42, F�LDY Lajos <fo...@rmki.kfki.hu> wrote:
>>> On Thu, 28 Aug 2008, noahh.schwa...@gmail.com wrote:
>>>> Hi,
>>>
>>>> I've been having problems with IDL accuracy. I'm trying to perform
>>>> calculations using the gamma function. The problem is that it grows
>>>> VERY fast! Performing this calculation in double (namely gamma(x)/
>>>> gamma(y) with x and y big) yields the result: NaN...
>>>> Would it be possible to use a program like 'Mathematica' (or any
>>>> other) and to plug it in my ILD program? Some kind of CALL_EXTERNAL
>>>> that is to say. If it is possible, how can I do it and what is the
>>>> best program to use?
>>>
>>>> Thanks,
>>>> Noah
>>>
>>> gamma(x)/gamma(y) => exp(lngamma(x)-lngamma(y))
>>>
>>> regards,
>>> lajos
>>
>>
>> lngamma works fine for my propose! Would you know if an equivalent
>> function exists for the beselk function? Something like lnbeselk?
>> beselk(x) for x>709 doesn't seen to work.
>
> Isn't 0 a good enough approximation?

If not, log(K(x,n))~ln(sqrt(!pi/(2*x)))-x for large x

Paolo

>
> Paolo
>
>
>> If not, I guess that I'll have to wait for the DLMs that add arbitrary
>> precision floating point...
>>
>> cheers,
>> Noah

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[Message index]

		IDL is not accurate enough! By: noahh.schwartz on Thu, 28 August 2008 09:30
		Re: IDL is not accurate enough! By: Haje Korth on Tue, 02 September 2008 05:20
		Re: IDL is not accurate enough! By: Kenneth P. Bowman on Fri, 05 September 2008 09:53
		Re: IDL is not accurate enough! By: MP on Fri, 05 September 2008 09:18
		Re: IDL is not accurate enough! By: Michael Galloy on Fri, 05 September 2008 08:57
		Re: IDL is not accurate enough! By: Kenneth P. Bowman on Thu, 04 September 2008 13:33
		Re: IDL is not accurate enough! By: pgrigis on Thu, 11 September 2008 07:44
		Re: IDL is not accurate enough! By: noahh.schwartz on Mon, 15 September 2008 02:37
		Re: IDL is not accurate enough! By: pgrigis on Thu, 11 September 2008 07:35
		Re: IDL is not accurate enough! By: pgrigis on Mon, 15 September 2008 06:40

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