# Homework Help: Number theory problem

1. Oct 30, 2007

### ehrenfest

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

$$\frac{1}{p} c(p,n) = (-1)^{n-1}/n (mod p)$$

I expanded that combination in every way I could think and I tried to use Wilson's Theorem and I couldn't get :(

2. Relevant equations

3. The attempt at a solution

2. Oct 30, 2007

### NateTG

That's p choose n, right?

Try writing the LHS out as a fraction with the stuff in the numerator as negative representatives. It should nicely cancel to give the result.

3. Oct 30, 2007

### ehrenfest

What do you mean "negative representatives"?

4. Oct 30, 2007

### Gokul43201

Staff Emeritus
-(p-1)/2, -(p-2)/2,..., -1 for odd p

Last edited: Oct 30, 2007
5. Oct 30, 2007

### matt grime

What is 1/n, or -1/n mod p supposed to mean?

6. Oct 31, 2007

### NateTG

Usually those would be the multiplicative inverses of n and -n respectively.

7. Oct 31, 2007

### matt grime

Usually? I beg to differ. Writing 1/n would indicate that the OP hasn't grasped what's going on. As would the fact there is an equals sign. I can't think of anyone who writes 1/2 mod 3 and not -1 0 it is incredibly bad notation. There is a difference from what I infer and what the OP ought to have written.

Last edited: Oct 31, 2007
8. Nov 1, 2007

### ehrenfest

What does OP stand for? Is that me?

I just realized that my book my book defines congruence as

$$x \equiv y \mod p$$

when x-y is a rational number whose numerator, in reduced form, is divisible by p.

So, it is like a generalized congruence or something...

Are there different rules for these generalized congruences?

I am not sure why what Gokul43201 wrote cancels nicely?