# Rubber band

1. Jan 8, 2009

### KFC

In the text, there is a rubber band model for 1D chain of N links. Assume each link of the rubber band is align horizontally either to the right or to the left. In the text, define $$N_+$$ as the number of links directed to the right and $$N_-$$ represents the number of links directed to the left. I think the total configuration of the system should be

$$\Omega = \frac{N!}{N_+!N_-!}$$

but it is

$$\Omega = \frac{2N!}{N_+!N_-!}$$

I wonder where is the "2" come from?