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AgnieszkaWielgus,JanZarzycki,FelicjaLwowandBartłomiejGolenko
definitesymmetricToeplitzmatrix.Therefore,parametrizationalgorithmsarefast
andsimplewhatisimportantfrompracticalpointofview.
However,inreal-lifewearefacedinmostcaseswithnon-stationarystochas-
ticprocesseswhosecovariancematricesarepositive-definiteHermitian,andthe
parametrizationproblemsolutionleadstothegeneralizedSchuralgorithmoffull
complexity,comparingtotheToeplitzcase[13],eveniftheprocessisnotnecessarily
totallynonstationary,anditscovariancematrixmaybeclosetotheToeplitzmatrix.
Therefore,considerationofstationaryversusnon-stationaryprocessesisnottoo
instructive.
ManyapproachestothenonstationarySchurparametrizationalgorithmcomplex-
ityreductionhavebeenproposed,tomentiontheMatrixExtensionProblem,and
theresultingstaircaseeliminationalgorithm,proposedin[12]andgeneralizedto
thenonlinearSchurparametrizationproblemofhigher-orderstochasticprocesses
[3,4,5,14].
Inanothervein,theapproachbasedonandfollowingfromtheconceptofU-
stationarity[2],resultsinahierarchicalclassificationofnonstationaryprocessesin
termsoftheirdistancefromstationarity.Thisconceptallowsalsoforthealgorithm
complexityreductionforU-stationaryprocessesforwhichU=2(n+1)(where
n=03...3#1)forthecovariancematrixofdimensions(#+1)(#+1).
Inthispaperweshow,forasubclassofsecond-orderstochasticprocesses,which
wecallP-stationaryprocesses,whosecovariancematricesareblock-Toeplitz(with
blocksofdimension(P+1)(P+1)(P=03...3#))[1],howtheirU-stationarity
isreflectedinthestructuresofthecorrespondingSchurparametrizationschemas.
ThepresentedapproachallowsustoreducethecomplexityoftheSchuralgorithm
forsecond-orderprocessesinauniformway.
Thispaperisorganizedasfollows:firstly,werecallthenotionofU-stationary
stochasticprocesses.Secondly,weintroduceaclassofP-stationaryprocessesand
showtheisomorphismbetweenthespaceofrandomvariablesandofcoemcient-
vectors.Next,weproposearecursiveGram-Schmidtorthogonalizationprocedureof
thebasesresultinginthegeneralizedLevinsonandSchuralgorithms,herederived
geometrically.Finally,weconsiderP-stationaryparametrizationandmodelingalgo-
rithms,usingtheKolmogorovisomorphism,andpresentstructuresoftheassociated
innovarionsandmodelingfiltersfornear-stationarystochasticprocesses.
102"-stationarysecond-orderstochasticprocesses
Let{3B3`}beaprobabilityspacewhereisanabstractsetofelementsl2
,B-af-algebraofBorelsubsets,and`aprobailitymeasureonB.Via
!2{3B3`}wedenoteaseparableHilbertspaceoff-measurablemappingsF:
!C,satisfyingØ
|F(l)|2`(dl)<.Weintroducetheinner-producton
!2{3B3`}as(F3E)
=Ø
F(l)¯
E(l)`(dl)=EF¯
Ewhere¯denotescomplex
conjugate,andEstandsfortheexpectationoperator.Thisinner-productinducesthe
normkFk2
=Ø
|F(l)|2`(dl)=E|F|2andmetricd(F3E)=kFEk.Let