# KarhunenLoeveSVDAlgorithm¶

class KarhunenLoeveSVDAlgorithm(*args)

Computation of Karhunen-Loeve decomposition using SVD approximation.

Available constructors:

KarhunenLoeveSVDAlgorithm(sample, threshold, centeredSample)

KarhunenLoeveSVDAlgorithm(sample, verticesWeights, threshold, centeredSample)

KarhunenLoeveSVDAlgorithm(sample, verticesWeights, sampleWeights, threshold, centeredSample)

Parameters:
sampleProcessSample

The sample containing the observations.

verticesWeightssequence of float

The weights associated to the vertices of the mesh defining the sample.

sampleWeightssequence of float

The weights associated to the fields of the sample.

thresholdpositive float, default=0.0

The threshold used to select the most significant eigenmodes, defined in KarhunenLoeveAlgorithm.

centeredSamplebool, default=False

Flag to tell if the sample is drawn according to a centered process or if it has to be centered using the empirical mean.

Notes

The Karhunen-Loeve SVD algorithm solves the Fredholm problem associated to the covariance function : see KarhunenLoeveAlgorithm to get the notations.

The SVD approach is a particular case of the quadrature approach (see KarhunenLoeveQuadratureAlgorithm) where we consider the functional basis of defined on by:

The SVD approach is used when the covariance function is not explicitly known but only through fields of the associated stochastic process : .

It consists in :

• approximating by its empirical estimator where and if the process is centered and otherwise, where ;

• taking the vertices of the mesh of as the quadrature points.

We suppose now that , and we note .

As the matrix is invertible, the Galerkin and collocation approaches are equivalent and both lead to the following singular value problem for :

(1)

The SVD decomposition of writes:

where we have , , such that :

• ,

• ,

• .

Then the columns of are the eigenvectors of associated to the eigenvalues .

We deduce the modes and eigenvalues of the Fredholm problem for :

We have:

The most computationally intensive part of the algorithm is the computation of the SVD decomposition. By default, it is done using LAPACK dgesdd routine. While being very accurate and reasonably fast for small to medium sized problems, it becomes prohibitively slow for large cases. The user can choose to use a stochastic algorithm instead, with the constraint that the number of singular values to be computed has to be fixed a priori. The following keys of ResourceMap allow one to select and tune these algorithms:

• ‘KarhunenLoeveSVDAlgorithm-UseRandomSVD’ which triggers the use of a random algorithm. By default, it is set to False and LAPACK is used.

• ‘KarhunenLoeveSVDAlgorithm-RandomSVDMaximumRank’ which fixes the number of singular values to compute. By default it is set to 1000.

• ‘KarhunenLoeveSVDAlgorithm-RandomSVDVariant’ which can be equal to either ‘halko2010’ for [halko2010] (the default) or ‘halko2011’ for [halko2011]. These two algorithms have very similar structures, the first one being based on a random compression of both the rows and columns of , the second one being based on an iterative compressed sampling of the columns of .

• ‘KarhunenLoeveSVDAlgorithm-halko2011Margin’ and ‘KarhunenLoeveSVDAlgorithm-halko2011Iterations’ to fix the parameters of the ‘halko2011’ variant. See [halko2011] for the details.

Examples

Create a Karhunen-Loeve SVD algorithm:

>>> import openturns as ot
>>> mesh = ot.IntervalMesher([10]*2).build(ot.Interval([-1.0]*2, [1.0]*2))
>>> s = 0.01
>>> model = ot.AbsoluteExponential([1.0]*2)
>>> sample = ot.GaussianProcess(model, mesh).getSample(8)
>>> algorithm = ot.KarhunenLoeveSVDAlgorithm(sample, s)


Run it!

>>> algorithm.run()
>>> result = algorithm.getResult()


Methods

 Accessor to the object's name. Accessor to the covariance model. Accessor to the object's id. Accessor to the object's name. Accessor to number of modes to compute. Get the result structure. Accessor to the process sample. Accessor to the weights of the sample. Accessor to the object's shadowed id. Accessor to the threshold used to select the most significant eigenmodes. Accessor to the weights of the vertices. Accessor to the object's visibility state. Test if the object is named. Test if the object has a distinguishable name. Computation of the eigenvalues and eigenfunctions values at nodes. setCovarianceModel(covariance) Accessor to the covariance model. setName(name) Accessor to the object's name. setNbModes(nbModes) Accessor to the maximum number of modes to compute. Accessor to the object's shadowed id. setThreshold(threshold) Accessor to the limit ratio on eigenvalues. setVisibility(visible) Accessor to the object's visibility state.
__init__(*args)
getClassName()

Accessor to the object’s name.

Returns:
class_namestr

The object class name (object.__class__.__name__).

getCovarianceModel()

Accessor to the covariance model.

Returns:
covModelCovarianceModel

The covariance model.

getId()

Accessor to the object’s id.

Returns:
idint

Internal unique identifier.

getName()

Accessor to the object’s name.

Returns:
namestr

The name of the object.

getNbModes()

Accessor to number of modes to compute.

Returns:
nint

The maximum number of modes to compute. The actual number of modes also depends on the threshold criterion.

getResult()

Get the result structure.

Returns:
resKLKarhunenLoeveResult

The structure containing all the results of the Fredholm problem.

Notes

The structure contains all the results of the Fredholm problem.

getSample()

Accessor to the process sample.

Returns:
sampleProcessSample

The process sample containing the observations of the process.

getSampleWeights()

Accessor to the weights of the sample.

Returns:
weightsPoint

The weights associated to the fields of the sample.

Notes

The fields might not have the same weight, for example if they come from importance sampling.

Accessor to the object’s shadowed id.

Returns:
idint

Internal unique identifier.

getThreshold()

Accessor to the threshold used to select the most significant eigenmodes.

Returns:
sfloat, positive

The threshold .

Notes

OpenTURNS truncates the sequence at the index defined in (3).

getVerticesWeights()

Accessor to the weights of the vertices.

Returns:
weightsPoint

The weights associated to the vertices of the mesh defining the sample field.

Notes

The vertices might not have the same weight, for example if the mesh is not regular.

getVisibility()

Accessor to the object’s visibility state.

Returns:
visiblebool

Visibility flag.

hasName()

Test if the object is named.

Returns:
hasNamebool

True if the name is not empty.

hasVisibleName()

Test if the object has a distinguishable name.

Returns:
hasVisibleNamebool

True if the name is not empty and not the default one.

run()

Computation of the eigenvalues and eigenfunctions values at nodes.

Notes

Runs the algorithm and creates the result structure KarhunenLoeveResult.

setCovarianceModel(covariance)

Accessor to the covariance model.

Parameters:
covModelCovarianceModel

The covariance model.

setName(name)

Accessor to the object’s name.

Parameters:
namestr

The name of the object.

setNbModes(nbModes)

Accessor to the maximum number of modes to compute.

Parameters:
nint

The maximum number of modes to compute. The actual number of modes also depends on the threshold criterion.

Accessor to the object’s shadowed id.

Parameters:
idint

Internal unique identifier.

setThreshold(threshold)

Accessor to the limit ratio on eigenvalues.

Parameters:
sfloat,

The threshold defined in (3).

setVisibility(visible)

Accessor to the object’s visibility state.

Parameters:
visiblebool

Visibility flag.

## Examples using the class¶

Validation of a Karhunen-Loeve decomposition

Validation of a Karhunen-Loeve decomposition

Viscous free fall: metamodel of a field function

Viscous free fall: metamodel of a field function

Metamodel of a field function

Metamodel of a field function