mu2

Mu2, written as mu with a subscript 2, is not a single universal concept. It is a notation used in multiple disciplines to designate either the second item in a sequence or a second statistical quantity, depending on the context.

In mathematics and statistics, μk commonly denotes the k-th moment of a random variable X about the origin, when that moment exists. Thus μ2 often represents the second raw moment: μ2 = E[X²]. In many texts, the second moment about the mean (the second central moment) is used as a convention as well, and is equal to the variance Var(X) when μ1 = E[X] and the second central moment is E[(X−E[X])²]. The relationship Var(X) = μ2 − μ1² shows how the second moment relates to the mean μ1 = E[X]. Because conventions vary, readers should verify the specific definition used in their source.

In physics and engineering, subscripts on μ often label parameters, components, or generations within a model. Consequently, μ2 may denote the second parameter, the second component of a vector of chemical potentials, a second mass parameter, or another second-order quantity within a given theoretical framework. The exact meaning is model-dependent and not standardized.

Other uses include labeling identifiers, versions, or gene/protein symbols in biological literature, where μ2 carries meaning only within the surrounding experimental or genomic context.

See also: mu, μ1, μ2 moments, second moment, variance, moment notation.