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The following exercise is a recap on probability and maths for statistical mechanics:

Maximize $$f(n_1,n_2,\, ...,\, n_M)=\dfrac{N!}{\prod_{j=1}^M n_j!}$$ subject to $$\sum_{j=1}^M n_j=N\ \text{ and }\ \sum_{j=1}^M e_jn_j=E,$$ with $e_j$ and $E$ constants.

Physical background: This problem can be understood as a problem of $M$ states and $N$ independent particles. The aim is to determine what is the distribution of particles among the different states that maximizes the number of microstates compatible with fixed values of $N$ and $E$.


Attempt:

I should impose $$\frac{\partial}{\partial n_i}(f(\mathbf n)-\lambda A(\mathbf n)-\beta B(\mathbf n))=0,\text{ for } i=\{1,\,...,M\},$$ where $A(\mathbf n)\equiv\sum_{j=1}^M n_j-N$ and $B(\mathbf n)\equiv \sum_{j=1}^M e_jn_j-E$, and so, $$\frac{\partial}{\partial n_i}\left(\dfrac{N!}{\prod_{j=1}^M n_j!}-\sum_{j=1}^M(\lambda+\beta e_j)n_j -\lambda N-\beta E\right)=0$$ $$\dfrac{N!n_i!}{\prod_{j=1}^M n_j!}\dfrac{\partial}{\partial n_i}\left(\frac{1}{n_i!}\right)-(\lambda+\beta e_i)=0,$$ but now I'm stuck here as I don't really know how to calculate the derivative of $1/n_i!$ or even how to proceed once I do it because the derivative should include digamma or gamma functions...

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    $\begingroup$ It might be easier to minimise the inverse so you don't have to use the quotient rule when differentiating. $\endgroup$ Mar 17 at 22:44
  • $\begingroup$ I guess $n_j, j=1,\dots,n$ should be integers, as well as $N$, $e$ and $E$. Could you check the question? Do you have the final solution or know why this problem is important? $\endgroup$
    – Amir
    Mar 18 at 17:06
  • $\begingroup$ @Amir I updated the background of the question just now. $\endgroup$
    – Conreu
    Mar 18 at 18:13
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    $\begingroup$ This is an integer programming problem. Derivative does not make any sense. Try to obtain all the possible feasible points. What do w know about $e_j$ and $E$? $\endgroup$
    – Red shoes
    Mar 18 at 18:46

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