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SIMPLICITY of CERTAIN GROUPS of DIFFEOMORPHISMS D. Epstein BULLETIN OF THE AMERICAN MATHEMATICAL SOCIETY Volume 80, Number 2, March 1974 SIMPLICITY OF CERTAIN GROUPS OF DIFFEOMORPHISMS BY JOHN N. MATHER1 Communicated by Shlomo Sternberg, October 1, 1973 D. Epstein has shown [1] that for quite general groups of homeo- morphisms, the commutator subgroup is simple. In particular, let M be a manifold. (In this note, we assume all manifolds are finite dimen­ sional, Hausdorff, class C00, and have a countable basis for their topology.) By a Cr+ mapping (resp. diffeomorphism) we mean a Cr mapping (resp. diffeomorphism) whose rth derivative is Lipschitz. Let Diff(M, r) (resp. Diff(M, r+)) denote the group of Cr (resp. Cr+) diffeomorphisms h of M such that there is an isotropy Ht of h to the identity, and a compact set K such that Ht(x)=x if x e M—K. Epstein showed in [1] that the commutator subgroup of Diff(M, r) (resp. Diff(M, r+)) is simple. Thurston announced in [4] that Diff(Af, oo) is simple. Let n=dim M. In this note we announce the following two results. THEOREM 1. If oo^r^.n + 1, then Diff(M, r+) is simple. r THEOREM 2. If ao^.r'^.n+.l, then FT ^ is (n + l)-connected. Here FY** denotes Haefliger's classifying space for codimension n foliations of class Cr+. These two theorems are closely related by results of Thurston (cf. [2], [4]). The case r= oo of these theorems is due to Thurston [4]. Here we outline a proof of Theorem 1. By Epstein's theorem, it is enough to show Diff(M, r+) is equal to its own commutator subgroup. A well-known argument shows that it is enough to prove the latter in n n the case M=R . Let A>1. Let D0={x eR : -2^x^2, l<y^/i}. For l<;/<;>7, let n Di = {xe R . -2 ^ Xj <; 2, 1 <Zj < U -2A ^ *, < 2A, i^j^ n). 00 Let a e Diff(/T, oo) be such that <x(x)=Ax if x e D0. Let p be a C real valued function on Rn, with compact support, such that O^p^l, n and p=l on D±. Let T,=exp(/>a/a^). Then T, G Diff(jR , oo), l^i^n. AMS {MOS) subject classifications (1970). Primary 58D05; Secondary 57D30. 1 This research was partially supported by NSF Grant GP-31359X-1 and a Sloan Foundation Fellowship. Copyright © American Mathematical Society 1974 271 License or copyright restrictions may apply to redistribution; see https://www.ams.org/journal-terms-of-use 272 J. N. MATHER [March LEMMA. There exists A0 such that the following holds. Let A^.A0. Let f r+ n be a C diffeomorphism of R 9 with support in D09 and sufficiently close to the identity (with respect to the Cr+ topology). Suppose oo>r^«+l. n Then there exist g09 gl9 • • • , gn9 Xl9 • • • , Xn e Diff(i? , r+) such that (1) v/gn*"1 = £o> 1 (2i) tigi-iTi^ = giTi9 1 ^ i «S n. PROOF THAT THE LEMMA IMPLIES THEOREM 1. It is enough to show that any diffeomorphism such as ƒ is a product of commutators, since any element of Diff(Rn, r+) is a product of conjugates of such diffeomor- phisms. Now if u e Diff(2£n, r+), let [u] denote its image in the commu­ tator quotient group. From (1), we get [/][gJ==[goL and from (2^, we get [gi-ii-lgil l£i<n. Hence [/] = 1. Q.E.D. OUTLINE OF THE PROOF OF THE LEMMA IN THE CASE r< oo. Let Bô n denote the subset of Diff(R 9 r+) consisting of g with support in D0 such that sup H/>•£(*) - D'g(y)\\l\\x -y\\<d. x^y For d>0 sufficiently small, and ƒ sufficiently near the identity, there exists a mapping <&.BÔ->BÔ such that if g G Bô and gn=®(g), then there n exist gl9 • • • , gn9 Xl9 • • • , Xn G Diff(R , r+) such that 1 (3) ofgvr = g0, 4 1 ( i) Aigi-iTiXi = gi-ri9 l^i^n. The mapping O is continuous with respect to the Cr topology. Since r Bô is compact with respect to the C topology, and convex, it follows from the Schauder-TychonofF fixed point theorem that O has a fixed point. But such a fixed point provides a solution of the equations (1), (2i). We can only sketch the idea of the construction of O. If u:Rn~>Rn vanishes outside a compact set, we define r \\u\\r+ = sup \\D u(x) - D'u(y)\\l\\x - y\\. x+y Then it is easy to see that if g0 is defined by (3), we have ||fo-id|U<^-'||/f-id||f+. Then we construct gl9 • • • , gn inductively. Supposing g^ has been defined, has support in Di9 and is near the identity, we construct gt to have support in Di+l9 to be near the identity and to satisfy \\gi - idlU < CA \\g^ - idlU. License or copyright restrictions may apply to redistribution; see https://www.ams.org/journal-terms-of-use 1974] SIMPLICITY OF CERTAIN GROUPS OF DIFFEOMORPHISMS 273 Here, C is a constant independent of A. By taking A sufficiently large, and ƒ sufficiently near the identity in relation to ô, we have that O maps B& into itself, where we define <£>(g)=gn. REFERENCES 1. D. B. A. Epstein, The simplicity of certain groups of homeomorphisms, Compositio Math. 22 (1970), 165-173. MR 42 #2491. 2. J. Mather, Loops and foliations, Proc. Internat. Conf. on Manifolds, Tokyo, 1973, (to appear). 3# 9 Commutators diffeomorphisms, Comment. Math, (to appear). 4. W. Thurston, Foliations and groups of diffeomorphisms, Bull. Amer. Math. Soc. (to appear). DEPARTMENT OF MATHEMATICS, HARVARD UNIVERSITY, CAMBRIDGE, MASSACHUSETTS 02138 License or copyright restrictions may apply to redistribution; see https://www.ams.org/journal-terms-of-use.
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