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1、IntroductiontoComplexityTheory{LectureNotesOdedGoldreichDepartmentofComputerScienceandAppliedMathematicsWeizmannInstituteofScience,Israel.Email:oded@wisdom.weizmann.ac.ilJuly31,1999IcCopyright1999byOdedGoldreich.Permissiontomakecopiesofpartorallofthisworkforp
2、ersonalorclassroomuseisgrantedwithoutfeeprovidedthatcopiesarenotmadeordistributedforprotorcommercialadvantageandthatnewcopiesbearthisnoticeandthefullcitationontherstpage.Abstractingwithcreditispermitted.IIPrefaceComplexityTheoryisacentraleldofTheoreticalCo
3、mputerScience,witharemarkablelistofcelebratedachievementsaswellasaveryvibrantpresentresearchactivity.Theeldisconcernedwiththestudyoftheintrinsiccomplexityofcomputationaltasks,andthisstudytendtoaimatgenerality:Itfocusesonnaturalcomputationalresources,andthee
4、ectoflimitingthoseontheclassofproblemsthatcanbesolved.Theselecturenotesweretakenbystudentsattendingmyyear-longintroductorycourseonComplexityTheory,givenin1998{99attheWeizmannInstituteofScience.Thecoursewasaimedatexposingthestudentstothebasicresultsandresearch
5、directionsintheeld.Thefocuswasonconceptsandideas,andcomplextechnicalproofswereavoided.Specictopicsincluded:RevisitingNPandNPC(withemphasisonsearchvsdecision);Complexityclassesdenedbyoneresource-bound{hierarchies,gaps,etc;Non-deterministicSpacecomplexity
6、(withemphasisonNL);RandomizedComputations(e.g.,ZPP,RPandBPP);Non-uniformcomplexity(e.g.,P/poly,andlowerboundsonrestrictedcircuitclasses);ThePolynomial-timeHierarchy;Thecountingclass#P,approximate-#PanduniqueSAT;Probabilisticproofsystems(i.e.,IP,PCPandZK)
7、;Pseudorandomness(generatorsandderandomization);TimeversusSpace(inTuringMachines);Circuit-depthversusTM-space(e.g.,AC,NC,SC);Average-casecomplexity;Itwasassumedthatstudentshavetakenacourseincomputability,andhencearefamiliarwithTuringMachines.Mostofthepres
8、entedmaterialisquiteindependentofthespecic(reasonable)modelofcom-putation,butsome(e.g.,Lectures5,16,and19{20)dependsheavilyonthelocalityofcomputationofTuringmachines.IIIIVStateofthesenot