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2021-11-27, 16:32   #23
charybdis

Apr 2020

23·73 Posts

Quote:
 Originally Posted by R. Gerbicz ...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

2021-11-29, 16:05   #24
Dr Sardonicus

Feb 2017
Nowhere

2×2,677 Posts

Quote:
 Originally Posted by R. Gerbicz Modified your idea, close to a pure combinatorial proof: The k=0 case is trivial, so assume that 0
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]

$(x\;+\;y)^{p}\;=\;x^{p}\;+\;y^{p}$

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

$(x\;+\;y)^{p^{n}}\;=\;x^{p^{n}}\;+\;y^{p^{n}}$

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

 2021-12-03, 13:29 #25 MattcAnderson     "Matthew Anderson" Dec 2010 Oregon, USA 17608 Posts more of this I am excited about this. *smile*
 2021-12-10, 17:33 #26 bur     Aug 2020 79*6581e-4;3*2539e-3 5·97 Posts Thanks a lot for all the replies, it will take me while to get through all that. By "reduced" I meant, there would be no smaller integer congruent to them mod p. I forgot about that smaller could also mean "negative".

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