# Seeking better explanation of some quantum stats formulae

• I
Gold Member

## Summary:

Reading Griffiths. He derives some formulas but I'm not following.

## Main Question or Discussion Point

In "Introduction to Quantum Mechanics", Griffiths derives the following formulae for counting the number of configurations for N particles.

Distinguishable particles...
$$N!\prod_{n=1}^\infty \frac {d^{N_n}_n} {N_n !}$$

Fermions...
$$\prod_{n=1}^\infty \frac {d_n!} {N_n!(d_n-N_n)!}$$

Bosons...
$$\prod_{n=1}^\infty \frac {(N_n+d_n-1)!} {N_n!(d_n-1)!}$$

In the above, ##N_n## stands for the number of particles in the nth state and ##d_n## stands for the degeneracy of the nth state. My confusion is not with the mathematics of combinatorics but only how it is being used here. Griffiths speaks of "picking particles" and "bins" and I'm only vaguely able to follow his argument. Furthermore, I can't search for these formulae online because they are not named in the text and I have been unsuccessful with searches for "quantum statistics", "configurations", etc. Can anyone here recognize these formulae and point me to a derivation of them? Thanks.

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• PeroK
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And found all your expressions -- with a little more explanation
Awesome. Thanks!

You should try working these expressions out on your own as they are very common in statistical mechanics.