One-dimensional with lattice constant (Problem)

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SUMMARY

The discussion focuses on calculating the average position of an atom in a one-dimensional lattice with a specified lattice constant, denoted as 'a'. The formula for the average position is derived from the expected value, represented as = ∑ x_i p_i. The specific equation for the position after N steps is given by x_n = (2n - N)a, where n represents the number of steps to the right. This formula is established based on the net displacement of the atom after N steps, accounting for both right and left movements.

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In question (a) Calculate the average position ?
https://en.wikipedia.org/wiki/Expected_value#Finite_case
##<x> =\sum x_ip_i##
I saw the answer and wondered.
## \bar{x} = <x> = \sum_{n=0}^{N} \frac{N!}{n!(N-n)!} x_n p^n q^{N-n}##
so ##x_n = (2n-N)a## a is lattice constant

why x_n = (2n-N)a ? why is that ?
 

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When after N steps of length a there were n steps to the right and (N-n) steps to the left, the distance xn traveled by the atom (starting at the origin x = 0) will be
xn = (na – (N-n)a) = (2n-N)a.
 
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