Show the number of arrangements that give an overall length of L = 2md

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The discussion centers on calculating the number of arrangements that yield an overall length of L = 2md, where L is defined as L = (N_+ - N_-)d. The factor of 2 in the expression N!/[(N_+)!(N_-)!] is crucial for accounting for the symmetry in arrangements, as both 2md and -2md represent the same physical state. For example, with N = 4 and m = 1, achieving a length of L = 2d requires 3 links in one direction and 1 in the opposite, leading to a doubling of arrangements due to this symmetry.

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Homework Statement
Show that the number of arrangements s that give an overall length
of L = 2md is given by g(N,m) = 2N!/[(N/2+m)!(N/2-m)!]
Relevant Equations
N = N_+ + N_-
L = 2md = (N_+ - N_-)d then 2m = N_+ - N_- , m is positive
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I know ## L = 2md = (N_+ - N_-)d ## then ## 2m = N_+ - N_- ##
So I can write ##N_+## and ##N_-## in term N and m
I don't understand the factor 2 multiplying in front of N!/[(N_+)!(N_-)!]
How does multiplication by the number "2" give a physical meaning?
 
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Another said:
I know ## L = 2md = (N_+ - N_-)d ## then ## 2m = N_+ - N_- ##
So I can write ##N_+## and ##N_-## in term N and m
I don't understand the factor 2 multiplying in front of N!/[(N_+)!(N_-)!]
How does multiplication by the number "2" give a physical meaning?
Suppose ##N = 4## and ##m = 1##: you want a length of ##L = 2d##. You need ##3## links in one direction and ##1## in the other. You can do that with ##3## links in the plus direction and ##1## link in the negative direction or vice versa. We can count the number of ways to get ##3## pluses and ##1## minus, then double it.
 
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2md and -2md are regarded same so doubled.
 
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