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NewWords&Expressions:conversely反之geometricinterpretation几何意义correspond对应induction归纳法deducible可推导的proofbyinduction归纳证明difference差inductiveset归纳集distinguished著名的inequality不等式entirelycomplete完整的integer整数Euclid欧几里得interchangeably可互相交换的Euclidean欧式的intuitive直观的thefieldaxiom域公理irrational无理的2.4整数、有理数与实数Integers,RationalNumbersandRealNumbersNewWords&Expressions:irrationalnumber无理数rational有理的theorderaxiom序公理rationalnumber有理数ordered有序的reasoning推理product积scale尺度，刻度quotient商sum和ThereexistcertainsubsetsofRwhicharedistinguishedbecausetheyhavespecialpropertiesnotsharedbyallrealnumbers.Inthissectionweshalldiscusssuchsubsets,theintegersandtherationalnumbers.4－AIntegersandrationalnumbers有一些R的子集很著名，因为他们具有实数所不具备的特殊性质。在本节我们将讨论这样的子集，整数集和有理数集。Tointroducethepositiveintegerswebeginwiththenumber1,whoseexistenceisguaranteedbyAxiom4.Thenumber1+1isdenotedby2,thenumber2+1by3,andsoon.Thenumbers1,2,3,…,obtainedinthiswaybyrepeatedadditionof1areallpositive,andtheyarecalledthepositiveintegers.我们从数字1开始介绍正整数，公理4保证了1的存在性。1+1用2表示，2+1用3表示，以此类推，由1重复累加的方式得到的数字1,2,3，…都是正的，它们被叫做正整数。Strictlyspeaking,thisdescriptionofthepositiveintegersisnotentirelycompletebecausewehavenotexplainedindetailwhatwemeanbytheexpressions“andsoon”,or“repeatedadditionof1”.严格地说，这种关于正整数的描述是不完整的，因为我们没有详细解释“等等”或者“1的重复累加”的含义。Althoughtheintuitivemeaningofexpressionsmayseemclear,incarefultreatmentofthereal-numbersystemitisnecessarytogiveamoreprecisedefinitionofthepositiveintegers.Therearemanywaystodothis.Oneconvenientmethodistointroducefirstthenotionofaninductiveset.虽然这些说法的直观意思似乎是清楚的，但是在认真处理实数系统时必须给出一个更准确的关于正整数的定义。有很多种方式来给出这个定义，一个简便的方法是先引进归纳集的概念。DEFINITIONOFANINDUCTIVESET.Asetofrealnumbersiscalledaninductivesetifithasthefollowingtwoproperties:(a)Thenumber1isintheset.(b)Foreveryxintheset,thenumberx+1isalsointheset.Forexample,Risaninductiveset.Soistheset.Nowweshalldefinethepositiveintegerstobethoserealnumberswhichbelongtoeveryinductiveset.现在我们来定义正整数，就是属于每一个归纳集的实数。RLetPdenotethesetofallpositiveintegers.ThenPisitselfaninductivesetbecause(a)itcontains1,and(b)itcontainsx+1wheneveritcontainsx.SincethemembersofPbelongtoeveryinductiveset,werefertoPasthesmallestinductiveset.用P表示所有正整数的集合。那么P本身是一个归纳集，因为其中含1，满足(a)；只要包含x就包含x+1,满足(b)。由于P中的元素属于每一个归纳集，因此P是最小的归纳集。ThispropertyofPformsthelogicalbasisforatypeofreasoningthatmathematicianscallproofbyinduction,adetaileddiscussionofwhichisgiveninPart4ofthisintroduction.P的这种性质形成了一种推理的逻辑基础，数学家称之为归纳证明，在介绍的第四部分将给出这种方法的详细论述。Thenegativesofthepositiveintegersarecalledthenegativeintegers.Thepositiveintegers,togetherwiththenegativeintegersand0(zero),formasetZwhichwecallsimplythesetofintegers.正整数的相反数被叫做负整数。正整数，负整数和零构成了一个集合Z，简称为整数集。Inathoroughtreatmentofthereal-numbersystem,itwouldbenecessaryatthisstagetoprovecertaintheoremsaboutintegers.Forexample,thesum,difference,orproductoftwointegersisaninteger,butthequotientoftwointegersneednottoneaninteger.However,weshallnotenterintothedetailsofsuchproofs.在实数系统中，为了周密性，此时有必要证明一些整数的定理。例如，两个整数的和、差和积仍是整数，但是商不一定是整数。然而还不能给出证明的细节。Quotientsofintegersa/b(whereb≠0)arecalledrationalnumbers.Thesetofrationalnumbers,denotedbyQ,containsZasasubset.ThereadershouldrealizethatallthefieldaxiomsandtheorderaxiomsaresatisfiedbyQ.Forthisreason,wesaythatthesetofrationalnumbersisanorderedfield.RealnumbersthatarenotinQarecalledirrational.整数a与b的商被叫做有理数，有理数集用Q表示，Z是Q的子集。读者应该认识到Q满足所有的域公理和序公理。因此说有理数集是一个有序的域。不是有理数的实数被称为无理数。Thereaderisundoubtedlyfamiliarwiththegeometricinterpretationofrealnumbersbymeansofpointsonastraightline.Apointisselectedtorepresent0andanother,totherightof0,torepresent1,asillustratedinFigure2-4-1.Thischoicedeterminesthescale.4－BGeometricinterpretationofrealnumbersaspointsonaline毫无疑问，读者都熟悉通过在直线上描点的方式表示实数的几何意义。如图2-4-1所示，选择一个点表示0，在0右边的另一个点表示1。这种做法决定了刻度。IfoneadoptsanappropriatesetofaxiomsforEuclideangeometry,theneachrealnumbercorrespondstoexactlyonepointonthislineand,conversely,eachpointonthelinecorrespondstooneandonlyonerealnumber.如果采用欧式几何公理中一个恰当的集合，那么每一个实数刚好对应直线上的一个点，反之，直线上的每一个点也对应且只对应一个实数。Forthisreasonthelineisoftencalledthereallineortherealaxis,anditiscustomarytousethewordsrealnumberandpointinterchangeably.Thusweoftenspeakofthepointxratherthanthepointcorrespondingtotherealnumber.为此直线通常被叫做实直线或者实轴，习惯上使用“实数”这个单词，而不是“点”。因此我们经常说点x不是指与实数对应的那个点。Thisdeviceforrepresentingrealnumbersgeometricallyisaveryworthwhileaidthathelpsustodiscoverandunderstandbettercertainpropertiesofrealnumbers.However,thereadershouldrealizethatallpropertiesofrealnumbersthataretobeacceptedastheoremsmustbededuciblefromtheaxiomswithoutanyreferencestogeometry.这种几何化的表示实数的方法是非常值得推崇的，它有助于帮助我们发现和理解实数的某些性质。然而，读者应该认识到，拟被采用作为定理的所有关于实数的性质都必须不借助于几何就能从公理推出。Thisdoesnotmeanthatoneshouldnotmakeuseofgeometryinstudyingpropertiesofrealnumbers.Onthecontrary,thegeometryoftensuggeststhemethodofproofofaparticulartheorem,andsometimesageometricargumentismoreilluminatingthanapurelyanalyticproof(onedependingentirelyontheaxiomsfortherealnumbers).这并不意味着研究实数的性质时不会应用到几何。相反，几何经常会为证明一些定理提供思路，有时几何讨论比纯分析式的证明更清楚。Inthisbook,geometricargumentsareusedtoalargeextenttohelpmotivateorclarityaparticulardiscuss.Neverthele