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Old 2021-11-27, 16:32   #23
charybdis's Avatar
Apr 2020

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Originally Posted by R. Gerbicz View Post
...furthermore: k*binomial(p,k)=p*binomial(p-1,k-1) is also true...
And we can do this combinatorially too: both are the way of choosing k elements from a set of p elements, where we label one of those k elements as "special".
We get k*binomial(p,k) by choosing the set of k elements and then picking the special element, and p*binomial(p-1,k-1) by first picking the special element and then choosing the other k-1 elements from the remaining p-1 elements of our set.

More famously, binomial(p-1,k-1)+binomial(p-1,k)=binomial(p,k) is easiest to see combinatorially: we partition the binomial(p,k) choices of k elements from the numbers {1,...,p} into two, depending on whether they contain 1 or not. binomial(p-1,k-1) is the number of choices containing 1, and binomial(p-1,k) is the number of choices not containing 1.

Last fiddled with by charybdis on 2021-11-27 at 16:35
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Old 2021-11-29, 16:05   #24
Dr Sardonicus
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Feb 2017

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Originally Posted by R. Gerbicz View Post
Modified your idea, close to a pure combinatorial proof:

The k=0 case is trivial, so assume that 0<k<p,
it is known: binomial(p-1,k-1)+binomial(p-1,k)=binomial(p,k) [for here you don't need that p is prime]
Of course! And for 0 < k < p, \frac{p!}{k!(p-k)!} clearly is divisible by p because the denominator is composed of factors less than p.

The result can be extended slightly. Since p divides binomial(p,k) for 0 < k < p when p is prime, we have the "freshman's dream" polynomial identity in Fp[x,y]


Repeatedly raising to the pth power, we see that in Fp[x,y] for any positive integer n,


which shows that binomial(pn,k) is divisible by p for 0 < k < pn.

Then the above argument shows that for p prime and any positive integer n,

{p^{n}-1\choose k}\equiv -1^k \;\pmod p

Last fiddled with by Dr Sardonicus on 2021-11-29 at 17:39 Reason: Omit repeated word
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