Electrical resistivity and conductivity results in the important concept of "quasineutrality", which says the density of negative charges is approximately equal to the density of positive charges over large volumes of the plasma ("n" = <Z>"n"), but on the scale of the Debye length there can be charge imbalance. In the special case that "double layers" are formed, the charge separation can extend some tens of Debye lengths. The magnitude of the potentials and electric fields must be determined by means other than simply finding the net charge density. A common example is to assume that the electrons satisfy the Boltzmann relation: Differentiating this
Positive and negative sets union of a sequence of positive or negative sets is also positive or negative; more formally, if ("A") is a sequence of positive sets, then is also a positive set; the same is true if the word "positive" is replaced by "negative". A set which is both positive and negative is a μ-null set, for if "E" is a measurable subset of a positive and negative set "A", then both μ("E") ≥ 0 and μ("E") ≤ 0 must hold, and therefore, μ("E") = 0. The Hahn decomposition theorem states that for every measurable space ("X",Σ) with a signed measure μ,