Expression of the configuration partition function

The configuration partition function is a function of the temperature which can be expressed as a sum of terms, each of which is the contribution that corresponds to a value of the degree of order s. We express this idea as follows  

Let us explicitly write each of these contributions, which are characterized by a value E of the energy expressed by equation [2.108], so by definition of the partition function  

For a given degree of order and a given temperature, as all the configurations have the same energy, the terms in the above sirni are all identical, and if we let g(5) denote the number of those configurations, the partition function of configuration will be  

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