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Topic: Advanced Permutations software
Replies: 2   Last Post: Feb 25, 2012 11:52 AM

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Wouter Meeussen

Posts: 5
Registered: 2/25/12
Re: Advanced Permutations software
Posted: Feb 25, 2012 11:52 AM
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>> I've never tried doing 250!/(30!*220!) in Excel

=GAMMALN(250+1)-GAMMALN(30+1)-GAMMALN(220+1)
gives 89,1728041505046 as its natural log,
in Mathematica it?s 533650325184459878135989793051885578200
(that?s 5.3365 10^38) with logarithm about 89.1728041505044882621285
so Excel is doing quite ok in floating point, but is limited to 10^15 for integers.

Wouter.

From: Esther Brink
Sent: Friday, February 24, 2012 1:30 PM
To: discretemath@mathforum.org ; benb@wcjc.edu
Subject: RE: Advanced Permutations software

Hardya,
It seems you have 250 locations, and need to choose 30 of them for A, while the other 220 automatically go to B.
If that's a fair analysis, you're looking for 250 C 30 or C(250, 30), the number of combinations of 250 objects, taken 30 at a time.
That calculation may be do-able on a standard TI-83 calculator, using the Math menu,then going to "Probability", etc. I've never tried doing it in Excel, but the calculation amounts to n!/[r!(n-r)!] where "!" means "factorial." So in this case you're looking at
250!/[30!*220!].
You don't indicate how accurate you want the answer to be. That could affect how much computational power you need. Would advise you to ballpark the most detailed criteria you have for your combinations before making your choices.
Hope this helps, but if not, just re-post. Some serious computer/calculator geeks are on here. Thanks.
Ben


> Date: Fri, 24 Feb 2012 06:09:10 -0500
> From: discussions@mathforum.org
> To: discretemath@mathforum.org
> Subject: Advanced Permutations software
>
> I am new to maths, but I think I know the basics. I am looking for some permutations software.
>
> Amongst other things I want to know a permutation like this:
>
> 250 distinct locations. At each location there may be an object of type A or type B. So I can say 2 to choose from, number chosen 250, order important, repetition allowed and I get 1.80925139e+75.
>
> However I know the ratio of A and B type objects is fixed, 30 A 220 B. I could say I have 250 distinct objects (already knowing for each if it is A or B) and just work out the combinations of locations, so n=250, r=250, order important, repeats no and get 3.23285626e+492. However I am not really bothred about the individual obejcts just the type at the location so many of these combinations would be for my purposes the same.
>
> So I want to find the number of combinations for any 30 A 220 B distributed around 250 locations. And I may have a few other similar things like this. However I'm not really interested in the maths so I am looking for some dedicated software which may provide results and equations, however most of the stuff I have seen so far only allow the basic parameters n, r, order?, repeat?.
>
> Does any one know of any software that would help me to do more complex cases such as the one I have suggested?
>
> Once I have got over that hurdle, I may want to think about ways to justify selecting a particular subset of such combinations/permutations as a valid sample.
>
> Any help or advice in this area would be greatly appreciated.




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