# Homework Help: Independent Sets of Cycles proof Involves Combinations!

1. Sep 19, 2007

### rbzima

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

Let $$C_n$$ Recalling our notation from class, we know that $$I_0(C_n) = 1$$ and $$I_1(C_n) = n.$$ Prove that for $$k > 1,$$

$$I_k(C_n)=\left(\frac{n}{k}\right)\left(\stackrel{n-k-1}{k-1}\right)$$

2. Relevant equations

$$I_k$$ implies number of independent sets of size k
$$C_n$$ implies the cycle with n vertices.
Therefore, $$I_k(C_n)$$ implies the number of independent sets of size k in some cycle with n vertices.
$$I_k\left(C_n\right)=I_k\left(C_{n-1}\right)+I_{k-1}\left(C_{n-2}\right)$$

3. The attempt at a solution

I've worked through the algebra to a point where it becomes rather disheartening, and it seems to be getting no where whatsoever. I've tried thinking of a combinatorial proof for this, however cannot come up with any scenario to think of or how it might be applied in this instance. If anyone can simply lead me in the right direction, that would be totally fantastic!

Last edited: Sep 19, 2007