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Factorials and 2 powers (Posted on 2021-08-13) Difficulty: 3 of 5
Find all pairs (k,n) of positive integers such that

k!=(2n-1)(2n-2)(2n-4)...(2n-2n-1)

No Solution Yet Submitted by Danish Ahmed Khan    
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re: Poor attempt at an explanation | Comment 3 of 5 |
(In reply to Poor attempt at an explanation by Jer)

I think you had it with the 2s:


On the RHS k! has approximately k factors of 2
On the LHS, the 2's grow very fast.  There are n(n-1)/2 factors of 2.

Note that here we are comparing like with like; number of factors of 2, versus number of factors of 2.

Thus for some n we need n>=n(n-1)/2, true only if n={0,1,2,3}.

If n>3, then RHS will always have more factors of 2 than LHS.

So the solutions given by Charlie, plus k=0, are exhaustive.

Edited on August 14, 2021, 10:06 pm
  Posted by broll on 2021-08-14 22:03:28

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