403

REDUCED K-THEORY AND THE GROUP G(D)

= D* I F* D'

R.HAZRAT Department of Mathematics, University oj Bielefeld, 99501 Bielefeld, Germany M. MAHDAVI-HEZAVEHI Department of Mathematical Sciences, Sharif University of Technology, P.O. Box 11965-9415, Teheran, Iran E-mail: [email protected] B. MIRZAII Institu.te for Stu.dies in Theoretical Physics and Mathematics, P.O. Box 19995-5746, Teheran, Iran Given a division algebra D with center Z(D) = F, put D* = D\{O} and denote by D' the derived group of D*. Using valuation theory and Platanov's results on reduced K- theory, we investigate some algebriac properties of the group G(D) = D* / F* D' which behave very much like the reduced Whitehead group SKI (D). In particular, we establish a stability theorem for G(D) and also compute it when D is a tame and totally ramified valued division algebra. Along this line, we construct some examples and settle some questions concerning the structure of D'.

In this note we initiate a study of the group Q(D) = D* IF* D ' , where D is a finite dimensional division algebra with center F. In [10] it is shown that if D is an algebraic division algebra over its center F, then for each element a E D, a[P(a):F] is in F* D'. This shows that G(D) is a torsion group and in particular if D is a finite dimensional division algebra of index n = i(D), then Q(D) is of bounded exponent dividing n. It turns out that the algebraic properties of Q(D) are closely related to those of SKI (D) and some functorial properties of SKI(D) can be carried over to Q(D). One can easily see that the following sequence is exact:

1 --+ J-Ln(F)IZ(D')

L SKI(D) ~ Q(D) ~ NrdD/p(D*)1 F*t& --+ 1,

(1)

where J-Ln(F) is the group of n - th roots of unity in F, and we observe that J-Ln(F) = F* n D(I) and ZeD') = F* n D', where l,g are canonical homomorphisms and h is induced by the reduced norm of D to F, and also D(I) is the kernel of the reduced norm and ZeD') the center of the derived group D'. This in effect shows that SKI(D) Q(D)

h '::::!.

n

NrdD/p(D*)IF* .

!::.

ILn(F)IZ(D') if and only if

404

One can also prove that some functorial properties of SKI (D) remains true for G(D). In particular, a similar proof as given in [2, p. 160] may be applied to show that if Dl and D2 are division algebras over a common centre F such that (i(Dd, i(D2» = 1, then G(Dl ®F D 2) ~ G(DI) x G(D2)' Throughout, we assume that D is a finite dimensional division algebra over a henselian valued field F = ZeD). Denote by VF, VD the valuation rings, MF, MD their maximal ideals, and F, D, the residue field and the residue division algebra of F and D, respectively. We also take r D , r F for the value groups, UD, UF for the groups of units of VD and VF, respectively. In this case we have the following natural exact sequence 1 -+ F*UD/F* D' -+ G(D)

-+

rD/rF -+ 1.

(2)

Furthermore, we assume that Z(D) is separable over F and CharF does not divide i(D) (Le. D is a tame division algebra). The standard reference book for valuation theory on division rings is [13]. The main object of this note is to prove that if D is a tame and un ramified valued division algebra over a henselian field F, then G(D) ~ G(D). From this fact we easily conclude a stability theorem for the group G(D). On the other extreme, if D is a tame and totally ramified division algebra, then we compute the group G(D). In particular, for arbitrary sequence of positive numbers nI, n2,"" nT' we construct a division algebra D such that G(D) ~ Znl X Znl X ••• X Znr X Zn r . It turns out that for a tame and totally ramified division algebra D the notion of cyclicity of D may be expressed in terms of the exponent of the group G(D). To be more precise, we show that a tame and totally ramified division algebra D over a henselian field F of index n is cyclic if and only if exp(G(D» = n. We start with the following PROPOSITION 1. Let D be a division ring with center F and L be a finite extension of F such that (i(D), [L : F]) = 1. Then the canonical homomorphism fJ L : G(D) -+ G(D ®F L) is injective. PROOF. LetaE D*suchthatfJL(aF*D') = 1. ThenaE L*(D®FL)'nD*. If we put [L : F1 = m, then L can be viewed as a subfield of the full matrix ring Mm(F). Let 9 : L -+ Mm(F) where 9 is the F- algebra monomorphism.

Consider the following sequence, D ---+ D®FL ~ D®FMm(F) ~ Mm(D). By theorem 3 and 4 of chapter 5, page 27 of [2], it is clear that, a ---+ a® 1 --+ a ® 1 ---+ aIm = Diag(a, a, ... , a), and for bEL we have 1 ® b ---+ 1 ® c ---+ c where g(b) = c E Mm(F).

405

Hence D®FL can be embedded in Mm(D). Now because a E L*(D®FL)' and a E D*, by considering it in Mm(D) we have diag(a, a, ... , a) = aIm =:: is where 1 E Mm(F) and S E SLm(D). Taking the Dieudonne determinant from both sides of the last equation, we obtain am E F* D' by corollary 1 and lemma 2, page 135 of [2]. On the other hand, having ai(D) E F* D' shows that a E F* D', which completes the proof. The following lemma is essential to our subsequent work. LEMMA. Let D be a tame division algebra of finite dimension over a henselian field F = Z(D) with index n, then (i) For each a E 1 + MD there is b E 1 + MF such that ab ED'. (ii) 1 + MD C (1 + MF)D'. PROOF. (i) Let a E 1 + MD. Since NrdD/F(a) = t E 1 + MF, using the Hensel's lemma for f(x) = xn - t gives c E 1 + MF such that en = t. Hence ac- 1 E D(l) n (1 +MD)' Applying the Platonov's congruence theorem (d. [12] and [5]), we obtain ac- 1 E D' which completes the proof. (ii) By part (i) we have 1 + MD ~ (1 + MF )D'. Now consider the groups fl = D* /1 + MD and P(D) = (1 + Mp)D'/(1 + M D). One can easily observe that P(D) = fl' and as Theorem 2.11 of [1] shows, the center of fl is F*(l + MD)/(1 + MD)' We claim that fl is not an abelian group, for otherwise UD = UF (1 + M D) which implies that D is totally ramified. Thus, D' = f.Le (F), where e = exp(rD/rF), (cf. the proof of Theorem 3.1 of [141). This shows that D' is not in 1 + MD and so fl is not abelian. Therefore 1 + MD i= (1 + MF )D'. THEOREM 1. Let D be a tame and unramified division algebra of finite dimension over a henselian field Z(D) = F. Then G(D) ~ G(D). PROOF. Consider the reduction map UD the following sequence:

D* ~ UD/l +MD ~ UD/(1

+ Mp)D'

-+

D*. By the Lemma, we have

~ UD/UpD' ~ F*UD/F*D'.

This induces the following epimorphism 1j;: D* /F* D' -+ F*UD/F* D'.

(3)

Since the valuation is unramified, we have Z(D) = F and D* = F*UD' There, fore, for a, b E D*, the element c = aba- 1 b- 1 may be written in the form c = Ci.{3Ci. -1 {3-1, where a and {3 E UD. This shows that K er'IjJ = 1, and the proof is complete.

Notice that if F is finite, then by (3) and (2), G(D) is finite. In particular for a tame division algebra over a local field, G(D) is finite and it is easy to See that exp(G(D)) = n = i(D). We observe that G(D) is infinite when D is the quaternion division algebra over rational numbers. Also, for global fields Which are finite extensions of Fq(x), if [D : F] = n 2 , then G(D) is infinite with e:x:ponent n since the reduced norm is ~urjective by Theorem 3.6 of [8]. This shows that, in contrast with the group SKI (D), we cannot always reduce the e:x:ponent of the group G(D). As an example, let D be a finite dimensional division algebra over its center F such that CharF does not divide i(D). Then D«x)) is a tame, henselian and unramified division algebra over F«x)) with the usual valuation. So G(D«x))) ~ G(D) which is a sort of stability theorem for G(D). In particular, take HR, the quaternion division algebra over real numbers R, then G(HR«X») is a trivial group. Now, put D = HR«x)). We then have D* j F* ~ D' j Z (D'), and it is easy to see that D* j F* is not a simple group a. COROLLARY 1. Let D be a tame and totally ramified division algebra of finite dimension over a heselian field F = ZeD) with index n, then D' = Z(D')«1 + MD) n D(l») and NrdDjF(D*)jF*n ~ fDjfF. PROOF. One can check that UD = UF (1 + M D ) and by the Lemma V D = UFD'. Since ZeD') = D' n UF, the first result follows. The rest is an easy exercise. REMARK. Proposition 3 of [4] shows that (1 + MD) n D(l) ~ [UD' D*]. In the totally ramified case, using the relations obtained in the course of the proof of Corollary 1, we have (1 + M D) n D(l) = [D', D*]. Now, if N is a normal subgroup of D* such that [D',D*] ~ N ~ F*UD then, N = Z(N)«1 +MD) n D(l»), where ZeN) = F* n N, (cf. [11]).

The next result shows that we can find Q(D) for some special cases. THEOREM 2. Let D be a tame and totally ramified division algebra of finite dimension over a henselian field F = ZeD) of index n. Then Q(D) ~ fDjfF and SKI (D) ~ f-Ln(F)jZ(D').

PROOF. Since D = F the first part follows from (3) and (2). For the second part, note that since UD = UFD' then D(l) = f.Ln(F)D', and thus aThis is in contradiction with the result obtained in the course of the proof of proposition 4 in [4] which states that (D®F K)' !:':': D'. It is not known whether the proposition 4 is true or not [6].

401

For cyclic division algebras, we may prove the following criterion. COROLLARY 2. Let D be a tame and totally ramified division algebra of finite dimension over a henselian field F = ZeD) with index n. Then D is cyclic if and only if exp(G(D)) = n. PROOF. The proof follows from Theorem 2 and the Example of section 1 of [9] and Corollary 2 of [8, p. 179]. PROPOSITION 2. Let D be a tame and totally ramified division algebra oj finite dimension over a henselian field F = ZeD) with index n. Then nIt! NrdD/p(D*) ~ F* ,where e = exp(rDjr p ). PROOF. Let x E D*. Suppose that the order of rp + vex) is s. Because of e = exp(rDjrp) we have sle. Since v(X S ) E rp, by Lemma 2 of 131, we obtain X S = Jus, where J E F* and u E 1 + Mp(x")' Set y = x/u, then yS = X SIuS = J. This shows that [F(y) : F] ~ s. By Ostrowski's Theorem [F(y) : F] = pCt[F(y) : F]lrp(y) : rFI, where CharF = p. Since the valuation is tame and also F(y) = F, we have [F(y) : F] = Irp(y) : rpl. But vex) = 'tI(y), so Irp(y) : rpl ~ s. This shows that [F(y) : F] = s. Now NrdD/F(x) == NrdD/p(y)NrdD/F(u). But U E 1 + MD and so NrdD/F(u) E 1 + MF· Also l+Mp = (l+Mp)n/s, hence NrdD/p(u) = In/s, where f E F*. By the tower formula for reduced norms, we obtain NrdD/p(y) = (NF(y)/F(y))n/s, which completes the proof. EXAMPLE. Let K be an algebraicly closed field, nl, n2,"" nr an arbitrary sequence of positive integers such that n = nl ... nr is not divisible by Char K. Let Dl = K«Xl)) and define 0'1 : Dl -)- Dl by the rule 0'1 (xI) = WIXl, where WI is a primitive nl - th root of unity. Now, let D2 = D 1 «X2,0't}) and set D3 = D 2«X3)). We define 0'2 : D3 -)- D3 by 0'2(X3) = W3X3, where W3 is a primitive n2 - th root of unity. In general, if i is even, set DHI = D i «Xi+l)) and if i is odd we define O'i : Di -)- Di by O'i(Xi) = WiXi, where Wi is primitive a n(i+l)/2 - th root of unity. Then Di+l = Di«XHl,O'i))' So we have D == D2r = K«Xb ... , X2r, 0'1, 0'3,··., 0'2r-d). By the Hilbert construction (d. [7]), F = ZeD) = K«X~l,X~t"",X~;_l'X~;)) with [D: F) = n 2. Finally, we define v : D* -)- r D = z2r by the rule veE kiX~l ... x~;) = (i 1 ,··· , i2r), where ii, ... ,i2r are the smallest powers of Xi'S in the lexicographic order. It is easily seen that v is a valuation on D with r D = z2r and r p = n 1 Z x n 1 Z X ... x nrZ x nrZ, Therefore rD/rF ~ Znl X Znl X ... X Znr X Znr' That is,

408

IrD : rFI = nin~ ... n; = [D: Fl· So D is tame and totally ramified. Hence, by Theorem 2, G(D) ~ Znl X Znl X •.. X Znr X Zn r . As we have seen above, there are examples of division rings such that D' /Z(D') is not simple. In general, if D is unramified over F then D' /Z(D') is not simple, for Z(D')(l + MD n D(l») cD'. As Corollary 1 shows, in the totally ramified case the equality occurs, hence we may pose the following question: QUESTION. Is D' /Z(D') a simple group in totally ramfied division algebra tame over its center.

Finally, it is believed that G (D) is rarely trivial, more precisely: CONJECTURE.

If G(D) = 1 then [D : F] =

22

Acknowledgments The authors would like to thank J. -Po Tignol for the communication concerning Proposition 2. References 1. M. Chacron, c-valuations and normal c-orderings, Ganad. J. Math. 41,

14-67 (1989). 2. P. Draxl, Skew Field, London Math. Soc. Lecture Note Ser. Vol 81 (Cambridge, Univ. Press. Cambridge, 1983). 3. P. Draxl, Ostrowski's theorem for Heneselian valued skew fields, J. Reine Angew. Math 354, 213-218 (1984). 4. Y.L. Ershov, Valuation of division ring and the group SKI (D), Dokl. Akad. Nauk SSSR 239, 768-771 (1978); English Translation in Soviet Math. Dokl. 19, 395-399 (1978). 5. Y.L. Ershov, Henselian valuation of division rings and the group SK1(D), Mat. Sb. 117, No. 1159 (1982); English Translation in ,Math. USSRSb. 45, 63-71 (1983). 6. Y.L. Ershov, Oral communication. 7. N. Jacobson, PI-Algebra, An Introduction, Lecture Note in Math. 441 (Springer, Berlin, 1975). 8. A.I. Kostrikin and I.R. Shafarevich, (Eds.), Algebra IX. (Finite groups of Lie type, Finite dimensional division algebra), (Springer, 1992).

409

9. D.W. Lewis and J.-P. Tignol, Square class groups and Witt rings of central simple algebras, J. Algebm 154, 360-376 (1993). 10. M. Mahdavi-Hezavehi, Extending valuations to algebraic division algebras, Comm. Alg. 22, (11), 4373-4378 (1994). 11. M. Mahdavi-Hezavehi, Commutator subgroups of finite dimensional division algebras, Rev. Roumaine Math. Pures Appl., to appear. 12. V.P. Platanov, The Tannaka-Artin problem and reduced K - Theory, Izv. Akad. Nauk. SSSR. Sere Math, 40, 227-261 (1976); English 'Iranslation in, Math USSR Izv. 10, 211-243 (1976). 13. O.F. Schilling, The Theory of Valuations, Amer. Math. Soc. Providence, R.I. (1950). 14. J.-P. Tignol and A.R. Wadsworth, Totally ramified valuations on finitedimensional division algebras, 1hln. Amer. Math. Soc. Vol. 302, No.1, 223-250 (1987).

reduced k-theory and the group g(d) = d* if* d' (1)

Hensel's lemma for f(x) = xn - t gives c E 1 + MF such that en = t. Hence ac-1 E ..... V.P. Platanov, The Tannaka-Artin problem and reduced K - Theory, Izv. Akad.

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