View source for Spectral decomposition of a random function

''spectral representation of a random function''

A representation of a [[Random function|random function]] (in particular, of a [[Stochastic process|stochastic process]]) by a series or integral with respect to some special system of functions, such that the coefficients in this expansion are pairwise uncorrelated random variables. A wide class of spectral representations of complex-valued random functions <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s0863601.png" />, <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s0863602.png" />, with zero mean value (that is, such that <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s0863603.png" />) can be written in the form

<table class="eq" style="width:100%;"> <tr><td valign="top" style="width:94%;text-align:center;"><img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s0863604.png" /></td> <td valign="top" style="width:5%;text-align:right;">(1)</td></tr></table>

where <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s0863605.png" /> is some set with a given system of  "measurable subsets"  (that is, a measurable space); <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s0863606.png" />, <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s0863607.png" />, <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s0863608.png" />, is a system of complex-valued functions of <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s0863609.png" />, depending on a parameter <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s08636010.png" />; <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s08636011.png" /> is an orthogonal random measure on <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s08636012.png" /> (with uncorrelated values, so that <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s08636013.png" /> for any two disjoint measurable subsets <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s08636014.png" /> and <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s08636015.png" />); and the integral on the right-hand side can be defined either as the mean-square limit of the corresponding sequence of integral Cauchy sums (), or, more generally, as a  "Lebesgue integral with respect to the measure Zdl"  (see , for example). According to the general Karhunen spectral representation theorem, a spectral representation (1) exists for a random function <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s08636016.png" /> if and only if the corresponding correlation function <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s08636017.png" /> can be written in the form

<table class="eq" style="width:100%;"> <tr><td valign="top" style="width:94%;text-align:center;"><img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s08636018.png" /></td> </tr></table>

where <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s08636019.png" /> is a non-negative measure on <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s08636020.png" />.

The best known class of spectral representations of random functions are the representations of stationary stochastic processes <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s08636021.png" /> as Fourier–Stieltjes integrals,

<table class="eq" style="width:100%;"> <tr><td valign="top" style="width:94%;text-align:center;"><img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s08636022.png" /></td> <td valign="top" style="width:5%;text-align:right;">(2)</td></tr></table>

where <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s08636023.png" /> is a random function of <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s08636024.png" /> with uncorrelated increments and <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s08636025.png" /> is either the real line <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s08636026.png" />, when the time <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s08636027.png" /> is continuous, or the interval <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s08636028.png" />, when <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s08636029.png" /> is discrete (and takes integral values). The existence of such a spectral decomposition follows from the general theorem of Khinchin (or Wiener–Khinchin) on the integral representation of the correlation function <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s08636030.png" /> (see [[Stationary stochastic process|Stationary stochastic process]]). This shows that any stationary stochastic process can be regarded as the superposition of mutually uncorrelated harmonic oscillations of various frequencies and with random phases and amplitudes. A spectral decomposition of similar form, but with <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s08636031.png" />-dimensional planar waves in place of harmonic oscillations, also exists for homogeneous random fields defined on a Euclidean <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s08636032.png" />-dimensional space <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s08636033.png" />, or on the lattice <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s08636034.png" /> of integer points in <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s08636035.png" />. In the case of a generalized stationary stochastic process, consider a linear functional <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s08636036.png" /> on the space <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s08636037.png" /> of infinitely-differentiable functions <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s08636038.png" /> of compact support satisfying the conditions

<table class="eq" style="width:100%;"> <tr><td valign="top" style="width:94%;text-align:center;"><img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s08636039.png" /></td> </tr></table>

<table class="eq" style="width:100%;"> <tr><td valign="top" style="width:94%;text-align:center;"><img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s08636040.png" /></td> </tr></table>

for all real <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s08636041.png" />, where <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s08636042.png" />. The functional <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s08636043.png" /> can be written in the form

<table class="eq" style="width:100%;"> <tr><td valign="top" style="width:94%;text-align:center;"><img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s08636044.png" /></td> <td valign="top" style="width:5%;text-align:right;">(3)</td></tr></table>

where

<table class="eq" style="width:100%;"> <tr><td valign="top" style="width:94%;text-align:center;"><img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s08636045.png" /></td> </tr></table>

is the Fourier transform of <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s08636046.png" />. Formula (3) follows from the fact that

<table class="eq" style="width:100%;"> <tr><td valign="top" style="width:94%;text-align:center;"><img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s08636047.png" /></td> </tr></table>

can be written in the form

<table class="eq" style="width:100%;"> <tr><td valign="top" style="width:94%;text-align:center;"><img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s08636048.png" /></td> </tr></table>

where the function <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s08636049.png" /> is a monotone non-decreasing spectral distribution function such that

<table class="eq" style="width:100%;"> <tr><td valign="top" style="width:94%;text-align:center;"><img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s08636050.png" /></td> </tr></table>

for some non-negative integer <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s08636051.png" /> (see ). If one takes the space of functions <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s08636052.png" /> to be some special space of entire analytic functions, then one arrives at a generalized stationary stochastic process <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s08636053.png" /> with exponentially increasing spectral function <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s08636054.png" /> (see ).

Spectral decompositions of special form also occur for homogeneous random fields on groups <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s08636055.png" /> and on homogeneous spaces <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s08636056.png" />. This is a consequence of Karhunen's spectral decomposition theorem together with certain well-known results on the general form of positive-definite functions (or kernels, which are functions in two variables) on the sets <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s08636057.png" /> and <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s08636058.png" />. In particular, for a homogeneous field <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s08636059.png" /> on an arbitrary locally compact Abelian group <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s08636060.png" />, the spectral representation of <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s08636061.png" /> has the form (1), where the role of the functions <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s08636062.png" /> is played by the characters <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s08636063.png" /> of <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s08636064.png" />, and the domain <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s08636065.png" /> of integration is the corresponding character group <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s08636066.png" /> (see, , , for example). Spectral representations of more complicated form also occur under fairly general conditions for homogeneous fields on non-commutative topological groups (see ). Finally, in the case of homogeneous fields on homogeneous spaces <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s08636067.png" />, the spectral decomposition of a field <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s08636068.png" /> involves the spherical functions (spherical harmonics) in the space <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s08636069.png" />, and the corresponding spectral representation of the correlation function <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s08636070.png" /> involves the zonal spherical functions (see , ). In particular, a general homogeneous field <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s08636071.png" /> on the sphere <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s08636072.png" /> in the three-dimensional space <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s08636073.png" /> admits a spectral representation of the form

<table class="eq" style="width:100%;"> <tr><td valign="top" style="width:94%;text-align:center;"><img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s08636074.png" /></td> <td valign="top" style="width:5%;text-align:right;">(4)</td></tr></table>

where

<table class="eq" style="width:100%;"> <tr><td valign="top" style="width:94%;text-align:center;"><img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s08636075.png" /></td> </tr></table>

are ordinary [[Spherical harmonics|spherical harmonics]] and the random variables <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s08636076.png" /> are such that <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s08636077.png" />, where <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s08636078.png" /> is the Kronecker delta. Corresponding to formula (4) there is an expression for the correlation function of the form

<table class="eq" style="width:100%;"> <tr><td valign="top" style="width:94%;text-align:center;"><img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s08636079.png" /></td> </tr></table>

where <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s08636080.png" /> is the angular distance between the points <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s08636081.png" /> and <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s08636082.png" />, and

<table class="eq" style="width:100%;"> <tr><td valign="top" style="width:94%;text-align:center;"><img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s08636083.png" /></td> </tr></table>

where the <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s08636084.png" /> are the [[Legendre polynomials|Legendre polynomials]]. Similarly, if <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s08636085.png" /> (where <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s08636086.png" /> are polar coordinates) is a homogeneous and isotropic field in the plane <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s08636087.png" /> (so that <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s08636088.png" />, where <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s08636089.png" /> is the Euclidean distance between the points <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s08636090.png" /> and <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s08636091.png" />), then the spectral representation of <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s08636092.png" /> can be written in the form

<table class="eq" style="width:100%;"> <tr><td valign="top" style="width:94%;text-align:center;"><img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s08636093.png" /></td> <td valign="top" style="width:5%;text-align:right;">(5)</td></tr></table>

where <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s08636094.png" /> is the Bessel function of order <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s08636095.png" /> (cf [[Bessel functions|Bessel functions]]). Here the <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s08636096.png" /> are random functions with uncorrelated increments such that

<table class="eq" style="width:100%;"> <tr><td valign="top" style="width:94%;text-align:center;"><img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s08636097.png" /></td> </tr></table>

where

<table class="eq" style="width:100%;"> <tr><td valign="top" style="width:94%;text-align:center;"><img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s08636098.png" /></td> </tr></table>

and <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s08636099.png" /> is a non-negative measure on the semi-axis <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s086360100.png" />. Corresponding to the spectral representation (5), there is the following expression for the correlation function <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s086360101.png" />:

<table class="eq" style="width:100%;"> <tr><td valign="top" style="width:94%;text-align:center;"><img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s086360102.png" /></td> </tr></table>

For further examples of spectral representations of homogeneous fields see –.

Spectral representations of random functions exist not only for stationary stochastic processes and homogeneous fields. For example, if <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s086360103.png" /> is an arbitrary stochastic process on the interval <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s086360104.png" /> with correlation function

<table class="eq" style="width:100%;"> <tr><td valign="top" style="width:94%;text-align:center;"><img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s086360105.png" /></td> </tr></table>

which is continuous in both arguments, then by virtue of Mercer's theorem in the theory of integral equations (cf. [[Mercer theorem|Mercer theorem]]) and Karhunen's spectral representation theorem, <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s086360106.png" /> admits a spectral representation of the form

<table class="eq" style="width:100%;"> <tr><td valign="top" style="width:94%;text-align:center;"><img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s086360107.png" /></td> <td valign="top" style="width:5%;text-align:right;">(6)</td></tr></table>

where the <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s086360108.png" /> and the <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s086360109.png" />, <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s086360110.png" /> are the eigenfunctions and eigenvalues of the integral operator on the function space with kernel <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s086360111.png" /> and <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s086360112.png" />. The spectral representation (6) of a stochastic process <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s086360113.png" />, defined on a finite interval, is the continuous analogue of the decomposition of a random vector into its principal components, which is often used in multivariate statistical analysis. It was obtained independently by a number of scientists (see , , for example) and is most frequently called the Karhunen–Loève expansion. Spectral representations of the form (6) are widely used in many applications, in particular in the theory of automatic control, where (6) and some related representations are often called canonical representations of stochastic processes (see ), and in meteorology and geophysics, where the term  "method of empirical orthogonal functionsmethod of empirical orthogonal functions"  is usually used, since the eigenfunctions <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s086360114.png" /> in practice must be approximately determined by empirical data (see , ).

A spectral representation of a random function <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s086360115.png" />, <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s086360116.png" />, can sometimes also mean a general representation of the form

(without the requirement that <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s086360117.png" /> is a random measure with uncorrelated values) into a certain standard (sufficiently simple) complete system of functions <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s086360118.png" />. This is most common in the case of decompositions of stochastic processes <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s086360119.png" /> with continuous time into functions <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s086360120.png" />, so that

reduces to . It follows from

that, in general, <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s086360121.png" /> can be written in the form

<table class="eq" style="width:100%;"> <tr><td valign="top" style="width:94%;text-align:center;"><img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s086360122.png" /></td> <td valign="top" style="width:5%;text-align:right;">(7)</td></tr></table>

where <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s086360123.png" /> is the complex-valued measure on the <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s086360124.png" />-plane defined by the relation

<table class="eq" style="width:100%;"> <tr><td valign="top" style="width:94%;text-align:center;"><img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s086360125.png" /></td> </tr></table>

Conversely, it can be easily shown that the fact that <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s086360126.png" /> can be written in the form (7) implies that there is a spectral representation

(see, for example, [[#References|[2]]]). Stochastic processes admitting a spectral representation , where <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s086360127.png" /> does not necessarily have uncorrelated increments, are called harmonizable stochastic processes. In this case, the complex measure <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s086360128.png" /> is called the spectral measure of <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s086360129.png" />, and the set of points of the <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s086360130.png" />-plane not having a neighbourhood of spectral measure zero is called the spectrum of the process <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s086360131.png" />. The spectrum of a stationary process <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s086360132.png" /> is concentrated on the line <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s086360133.png" />. Under fairly general conditions, periodically correlated (or periodically non-stationary) stochastic processes <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s086360134.png" /> (which have the property that

<table class="eq" style="width:100%;"> <tr><td valign="top" style="width:94%;text-align:center;"><img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s086360135.png" /></td> </tr></table>

<table class="eq" style="width:100%;"> <tr><td valign="top" style="width:94%;text-align:center;"><img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s086360136.png" /></td> </tr></table>

for some <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s086360137.png" /> and any integer <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s086360138.png" />) are also harmonizable. The spectra of such processes are concentrated on the set of straight lines <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s086360139.png" />, <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s086/s086360/s086360140.png" /> (see [[#References|[8]]] or [[#References|[11]]]).

====References====
<table><TR><TD valign="top">[1]</TD> <TD valign="top">  K. Karhunen,   "Ueber lineare Methoden in der Wahrscheinlichkeitsrechnung"  ''Ann. Acad. Sci. Fennicae Ser. A. Math.-Phys. (I)'' , '''37'''  (1947)  pp. 3–79</TD></TR><TR><TD valign="top">[2]</TD> <TD valign="top">  Yu.A. Rozanov,   "Spectral analysis of abstract functions"  ''Teor. Veroyatnost. i Primenen.'' , '''4''' :  3  (1959)  pp. 292–310  (In Russian)</TD></TR><TR><TD valign="top">[3]</TD> <TD valign="top">  I.M. Gel'fand,   N.Ya. Vilenkin,   "Generalized functions. Applications of harmonic analysis" , '''4''' , Acad. Press  (1968)  (Translated from Russian)</TD></TR><TR><TD valign="top">[4]</TD> <TD valign="top">  T. Onoyama,   "Note on random distributions"  ''Mem. Fac. Sci. Kyushu Univ. Ser. A'' , '''13'''  (1959)  pp. 208–213</TD></TR><TR><TD valign="top">[5a]</TD> <TD valign="top">  A.M. Yaglom,   "Second-order homogeneous random fields" , ''Proc. 4-th Berkeley Symp. Math. Stat. Probab.'' , '''2''' , Univ. California Press  (1961)  pp. 593–622</TD></TR><TR><TD valign="top">[5b]</TD> <TD valign="top">  A.M. Yaglom,   "Spectral representations for various classes of random functions" , ''Proc. 4-th All-Union Mat. Konf. 1961'' , '''1''' , Leningrad  (1963)  pp. 250–273  (In Russian)</TD></TR><TR><TD valign="top">[6]</TD> <TD valign="top">  E.J. Hannan,   "Group representations and applied probability" , Methuen  (1965)</TD></TR><TR><TD valign="top">[7]</TD> <TD valign="top">  M.I. Yadrenko,   "Spectral theory of random fields" , Optim. Software  (1983)  (Translated from Russian)</TD></TR><TR><TD valign="top">[8]</TD> <TD valign="top">  A.M. Yaglom,   "Correlation theory of stationary and related random functions" , '''1–2''' , Springer  (1986)  (Translated from Russian)</TD></TR><TR><TD valign="top">[9]</TD> <TD valign="top">  V.S. Pugachev,   "Theory of random functions and its application to control problems" , Pergamon  (1965)  (Translated from Russian)</TD></TR><TR><TD valign="top">[10a]</TD> <TD valign="top">  M.I. Fortus,   "Method of empirical orthogonal functions and its meteorological applications"  ''Meteorologiya i Gidrologiya'' , '''4'''  (1980)  pp. 113–119  (In Russian)</TD></TR><TR><TD valign="top">[10b]</TD> <TD valign="top">  H. von Stroch,   G. Hannoschöck,   "Statistical aspects of estimated principle components (EoFs) based on small sample sizes"  ''Climate Appl. Meteor.'' , '''24'''  (1985)  pp. 716–724</TD></TR><TR><TD valign="top">[11]</TD> <TD valign="top">  S.M. Rytov,   "Introduction of statistical radiophysics" , '''1. Random processes''' , Springer  (1988)  (Translated from Russian)</TD></TR></table>



====Comments====
Spectral decomposition of not necessarily stationary random functions is given in [[#References|[a1]]].

====References====
<table><TR><TD valign="top">[a1]</TD> <TD valign="top">  A.G. Ramm,   "Random fields: estimation theory" , Longman &amp; Wiley  (1990)</TD></TR><TR><TD valign="top">[a2]</TD> <TD valign="top">  A.V. Ivanov,   N.N. Leonenko,   "Statistical analysis of random fields" , Kluwer  (1989)  (Translated from Russian)</TD></TR><TR><TD valign="top">[a3]</TD> <TD valign="top">  J.L. Doob,   "Stochastic processes" , Wiley  (1953)</TD></TR><TR><TD valign="top">[a4]</TD> <TD valign="top">  D.R. Cox,   H.D. Miller,   "The theory of stochastic processes" , Methuen  (1965)</TD></TR><TR><TD valign="top">[a5]</TD> <TD valign="top">  M.S. Bartlett,   "An introduction to stochastic processes" , Cambridge Univ. Press  (1978)</TD></TR></table>