# A division problem.

1. May 30, 2009

### zetafunction

1. The problem statement, all variables and given/known data

the idea is to prove wether a prime 'p' divides the quantities $${p \choose k}$$

for k=01,2,3,....,p-1

2. Relevant equations

3. The attempt at a solution

i have tried by inspection for small primes 3,5,7,11,13,17,...... but can not guess a simple solution , the idea is to see if for p prime , the Cyclotomic polynomial

$$x^{p}+..........+x+1$$ for p prime.

2. May 30, 2009

### slider142

Trying more general circumstances, where p is any prime, we get the simpler question of whether the product (p-1)...(p-n) is divisible by n+1 for all n <= p - 1.
Ie., for n = 1, we question whether p-1 is divisible by 2, which is trivial. (p-1)(p-2) divisibility by 3 can be seen as well with a little more care.
There is a basic divisibility theorem about these products that you may have already proven earlier. If not, it is not difficult to prove by a little inspection.

3. May 30, 2009

### tiny-tim

Hi zetafunction!
If p is prime, how can it not divide pCk (except for k = 0) …

there's a p on the top, and nothing larger than p-1 on the bottom.

4. May 30, 2009

### slider142

This question may only be trivial in retrospect. ;) The quotient $\frac{(r-1)!}{(r-k)!k!}$ trivially simplifies to $\frac{(r-1)\cdots(r-k-1)}{k!}$ which depends on k! successfully dividing the product on the top. This is not generally true when r is not prime. Ie., 8 does not divide 8C4.
It is then up to the OP to show why this is true for primes, but not necessarily composites.