C H a P T E R PI^Eliminanes in This Chapter We Collect, for The
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C H A P T E R PI^ELIMINAnES In this chapter we collect, for the convenience of the reader, all the known concepts and results i^loh are required in the subsequent chapters. These are divided into sections bearing titles of the topics to which they relate. 1.1. Fitlds with valuation Let Gi toe €m ordered group, nultiplicatively written. Let, further-f- toe an associative operation on G\ such that the distributive laws (1.1.1) ac +• ^c z^(gu+&)<L, (1.1.2) -Ca/4fr<^^<= CCOu^t)^ hold for all CL, -6- , C , in Gi . We shall adjoin a zero element to <?, i.e. we form QU{o] such that (1.1.3) OJ-O- OCL= 0 for all OU € Gi^ (1.1.4) 0 < (h for all a, & G. - 2 - DEFINITION 1.1.1 (BACHMAN [3], p. 72, Definition 2.2). A valuation of a field K is a map 1 \". K-^ 6i U{oJ, \^ei» G iB an ordered group provided (1.1.5) for a G K , (Cbl = 0 if and only L-f CL = Oj (1.1.6) for CO, ^€ K . 10.^1 = la.1 l-6-( ; (1.1.7) for a.-^GK, lCL+-tl^ l^l+l^l. It is always possible to introduce an additive operation in an ordezed group Gi by defining (1.1.8) OJ-^ (^ = 'Yv^doc. CCb , ^) for a/,^G6t. It is easily seen that this -p is associative and that the distributive laws are satisfied, Further, condition (1.1.7) of Definition 1.1.1 becomes (1.1.9) \ch~{-J^\ <, TVLC^ac Clcol , \^0. The valuation in this case will be called non-archlmediaB. Ve shall assume throughout that <j\ ^ C&) • DEFINITION 1.1.2 (BACHMAN SO p.77, Definitions 3.1t 3.2). A subgroup H of <?i , where G] is an ordered group is called isolated if ^ eH, and ^ < 00 ^ (L^ where CL e G\ , imply CU€|-|. The order type of the set of all isolated subgroups of (jj distinct from G\ is called the rank of 6i . DEFINITION 1.1.3 (BACHMAN [3l p. 77, Definition 3.3) If I I is a non-archimedian valuation of a field K into ^^U'foj where Gr is an ordered group, then the rank of \ I la the rank of Gj - 3 - PHOPOSITION 1.1.1 (BACHMAN [3] p. 81, Th«oi»m 3.4). An oTiered group G\ of rank 1 is order Isomorphic to a subgroup of the additive group Tl of real numbers. RBMAHK 1.1.1. In riew of Proposition 1.1.1, in consider ing valuations of rank 1, we may assume that the ordered group Gt is a subgroup of the additive group of real numbers with the usual ordering. We shall be concerned with valuations of rank 1 only. A valuation of rank 1 may thus be defined as follows. DEFINITIOH 1,1,4 (BACHMAH [3] p.5, Definition 2,1). A valuation of rank 1 of a field K is a mapping | I : from K into the reals 1\ such that for all CL , ^ e k ^ (1.1.10) \(Xj\7'yO and \ou\-0 if and only if CO = 0 ; (1.1.11) 1 OuL[ - fCLl 1^1 •. (1.1.12) I CL-f ^1 ^ la.l+1^1.; We shall generally omit, for the sake of brevity, mention ing that the valuation is of rank 1 explicitly and simply speak of such a function as a valuation, it being understood that such a map is always into the reals'Bv. K is called a field with a valuation or a valued field. If (1.1.12) is satisfied in the stronger foim (1.1.13) I Cb-+- Qy\ < -ma^c c \ou\ ,1^0, a., ^ e K^ then it is called a non-archimedian valuation. - 4 - We give some examples of non-aichlmedlan valuations. EXAMPLE 1.1.1. Por any field K , we define \0\~0 and \aj\=-\ toT all 0/4^0 in K . This is called the trivial valuation on k . Valuations other than the trivial valuation are called non-trivial valuations. EXAKPLS 1.1.2, Let Q denote the field of rational numbers, c a fixed real number such that 0-cc< I and p a fixed prime number. If x is any rational number other than 0 we can write where CL , -^ , oC are integers, [^ "fa, ^ j't-S- • We define (1.1.14) l^lb= C°^ O/^ct 10((,= 0. It is known (BACHMAH [3l , p.l) that ( (. is a non- archimedian valuation on Q and is called the \:> -adic valuation of Q. EXAMPLE 1,1.3. Let K be any field and KLt]be the ring of polynomials over K . If f>Cfc;i8 an irreducible polynomial in K Lb] then we define (1.1.15) locctM.= , ? where (XCb), -^Cb)Gl<Cb3, \=>'\ CL . f^t"^* ®"^ C is a m» ^ mm fixed real niunber In ( 0 » X )• (1.1»15) gives a non- archimedian valuation on KUb] and is called a ^Cb)- ~adio valuation of KLtUCsee Bachman [5] p.14). It follovnB immediately fiom Definition 1.1.1 that a valuation satisfies the following conditions (1.1.16)1 11 = I • ; (1.1.17)1-11 =^i ^ (1.1.18) 10.^1= I 0.11^1-' ; (1.1.19) |i<5^l-l^i|^ \aj-U. PPOPOSITION 1.1.2 (BACHMAH [?] pp.7» 8). If | | is a non>archiffledian valuation and if Icol > \£>,\ then \(Xy+4>-\~\(l>\ More generally, we hare . COH)ItLABY 1.1.1. For a non-archimedian valuation we have and I a,+ a2_+--- + clw| = I Ob, I -il^ 10.^1^ 10/(1 -for j.= i-.5,-.Ti The Corollary is a consequence of finite induction an Yl using Proposition 1.1.2. * We denote by 1 the unit element of K as also the real number 1, with, of course, no room for confusion. - 6 . The following characterization of a non-aiohlffledlan valuation will be freely used in the sequel. PROPOSITION 1.1.3 (HAMANATHAH [23] , p.204). A Taluation | | on a field K is non-arohimedian if and only If (1.1.20) 1 Til ^ 1 for eveiy Integer yi e K * ThlB shows immediately that COK)LLA!?y 1.1,2 (RAMANATHAN §3] , p. 204). All the valuations of a field of characteristic j=>=|=0 are non- archimedian. We continue to assume that I i is a non-aichimedian valuation and we consider the set Sy of all cu€ K such that I CU| ^ 1 • ^®* ^ denote the set of all cue K such that lal< 1- PHOPOSITIOH 1.1.4 (BACHMAN [3] , p. 8. Theorem 2.3). If I ( is a non-archimedian valuation, then the set V is a zing with unit element and 'P is the luiique maxiaal ideal of V and hence is a prime ideal. DEPIHITIOH 1.1.5. The ring V (defined above) is called the valuation ring associated with the non-archimedian valuation | j • The field \^ is called the associated residue class field. * Throughout we write \r\\ v^ere n is an integer, for Ini/ (1 being the unit element of K ). - 7 - (1.1.11) of Definition 1.1.4 showe that CL -^ (Ctl 1» a homomorphism of the multiplicative group K of non-sseio elements of K into the multiplicative group of positive reals. This homomorphic image is a group | K ( which we call the value group of K (see MMA.HATHAN [23l p,213), PEFINITION 1.1.6. The valuation | j is called discrete or dense according as the value group of K is discrete or dense. HEMAPK 1.1.2. A valuation is either discrete or dense, A valued field K can be viewed as a metrLo space with the distance function P defined by (1.1.21) pccb,t)= |a-^l satisfying (1.1.22) pCa..-6L);^0 and PCa,6.) = 0 if ana only If (X - ^ ', (1.1.23) pca,^)= pc-^.o,): (1.1.24) Pca,^) ^ PCa.,c)-i-pCc,l), Ca,4,c£K) If, in addition* K is a non-arohlmedian valued field then this distance function P can be easily seen to satisfy the following ultrametrlc Inequality (1.1,25), the stronger form of (1.1.24) (1.1.25) ?COL,t) ^ 7na:>c (pCCL, c) , pcc, 6.)) • Thus a valued field K is a metric space. - 8 - THEOREM 1.1.1 (BHJHAT [>] , p.18). Let K be a field with a non-trivial non-archimedian raluation. Then the following conditions axe equivalent. (1.1.26) K is locally-coapact. (1.1.27) The valuation ring V of K is compact, (1.1.28) K is ^ complete metric space. ( | is a discrete valuation and the residue class field is a finite field. The above metric p gives rise to the notion of conver gence in K as follows. If {jJir\,] is a sequence of elements of K then it is said to converge to the element CU in K » i^f for every real number £. > 0 , there exists an integer Nce) such that la^-Cul < ^ for -rv y^ NCe) . In terms of the metric, more explicitly, this means that PCdyu, Cl/)-> 0 , •Xi _^ CO • The sequence •[Q./>^] is said to be a Cauchy sequence if for every real number 6. > 0 there exists an UC&) such that \Ci'y[-(X'Yv\\ <S whenever ru.YW ;:> NCE-j. In tenas of the metric this means PCd^, OL'TA) -> 0 as n,'rv\ -> oo , independently of each other. However, it is clear from the following proposition that the independence of VL , rrt in the above definition is of no significance in a non- archimedian valued field.