Re: Need permutation help for n!/(n-r)!

[wraithx@morpheus.net]

On 6/25/2016 7:53 AM, chandra sekaran wrote:
> Hi Bill
>
> I have not put the question properly.
>
> (18:08) gp > 50!/(50-3)!
> %3 = 117600
>
> Out of 117600 ,  some number following permutation will fall. I want to find the
> number.
> [38, 40, 41] , [2,3,5], [10,11,12], [12,11,10]
> or Find the three number permutation which will fall on 2000.
>
> If  n grows bigger( say 600! ) fast finding three number set and reverse  is
> possible?
>
> Thanks
>
>
> Chandrasekaran B

You will need to define what order you want to put these elements in.  For 
example [2,3,5], could be obtained with:
[1,2,3] #1
[1,2,4] #2
[1,3,4] #3
[2,3,4] #4
[1,2,5] #5
[1,3,5] #6
[1,4,5] #7
[2,3,5] #8 *Found
Found on the 8th element.

Or, is it:
[1,2,(3-50)] #1-48
[1,3,(4-50)] #49-95
[1,4,(5-50)] #96-141
...
[1,49,50] #1176
[2,3,4] #1177
[2,3,5] #1178 *Found
Found on the 1178th element.

Also, do the elements of interest for you range from 0-49, or from 1-50? (the 
above examples use 1-50)

-David C.