Wigner quasi-probability distribution is the main tools used in the formulation of Quantum mechanics on phase space which is equivalent to usual formulations. This distribution can have negative values at some region, unlike usual probability distributions is non-negative.
I'm new about this subjects, and so I searched about this distribution. I found this sentences in Wikipedia.
Regions of such negative value are provable (by convolving them with a small Gaussian) to be "small": they cannot extend to compact regions larger than a few $ħ$, and hence disappear in the classical limit. They are shielded by the uncertainty principle, which does not allow precise location within phase-space regions smaller than $ħ$, and thus renders such "negative probabilities" less paradoxical.
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Hence, I would like to know where this result is proven.