(3.1) We need some easy results concerning the exponential map: Let M be a linear algebraic group and m its Lie algebra. Let M = Spec (C^ ^ where (9^^ is the local ring of M at the identity e and C^e lts completion, and similarly define m. The exponential map from m to M is a local analytic isomorphism, and thus it induces an isomorphism of affine schemes exp: m ^ M.
Define m(B),, m(B),, M(B), and M(B), as in 0.6. It is easy to see that the canonical morphism m - ^ m induces an isomorphism m(B),^m(B), for r ^ l , and similarly M(B),^M(B), for r ^ l . If (pem(B),, then e x p ° ( p : B - ^ M lies in M(B),. It is obvious that we have the following result:
(3.2) Proposition. — Let M be as above, r^ 1.
(1) exp: m (B\ -> M (B\ is a bijection.
(2) If M is a r-group, then the exponential map is Y-equivariant, and exp ^(B)^ N1(6)^.
(3.3) Let M be a F-group. We show that M has the approximation property if Rad(M°)=Rad^(M0), e.g., if M is semisimple.
Recall that L° is special (IV. 2.6), so we really only need to consider the approximation property for special groups. However, the general case is no harder.
(3.4) Lemma. - Let r> 1 and let K be a finite central Y-subgroup of the Y-group M.
(1) The canonical map M -> M/K induces isomorphisms M (B)^ (M/K) (B)^, M (6)^ ^ (M/K) (B)^ and M (B)^ ^ (M/K) (B)^.
(2) M has the approximation property if and only if M/K does.
Proof. - Applying H^ (B, -) to the exact sequence 1 -^ K ^ M -. M/K -^ 1
we obtain the exact sequence
1 -^ K^K(B) ^ M (B) -^ (M/K) (B) ^ 1
since H,\(B,K) is trivial. It follows that M (B),^ (M/K) (B),, and similarly for B replaced by B and B. (The B case also follows from 3.2). We have proved (1), and (2) is immediate from (1). •
(3.5) Theorem. - Let M be a T-group such thai Rad(M°)=RadJM°). Then M has the approximation property.
Proof. - By 2 . 1 (2), 2.2, 2 . 3 and 2 . 5 we may reduce to the case that M is connected and semisimple. Let B==T x U be a Borel subgroup of M, where T is a maximal torus and U a maximal unipotent subgroup of B, all being F-subgroups of M (see 2.5). It suffices to show:
(^) Given g (s) e M (B)^, r > 0, there is a g (s) e M (B^ such that g(srlg(s)eM(B)^,.
Choose an embedding M c GL^ for some n. Then we may consider m as a subalgebra of gl^. Let g (s) e M (B)^ with g (s) = I + ^ A + 0 (V +1). Then, for z e C small, z i - ^ I + z A i s a curve in GL,, which is tangent to M at I, hence Aem. Clearly /"A is r-invariant. Hence (^) follows from
(^) Let A e m such that s' A ^ r-invariant. Then there is a g(s)eM(B)^ with g(s)=l-^srA+0(sr+l).
Note that, by Lemma 3.4, properties (^) and (^) are really properties o f m alone and do not depend upon the various possible M's.
Let Y denote a generator ofF. We consider the action o f F on m: Clearly we may reduce to the case where m = m i ® . . . ® m^, each m, is simple and isomorphic to mi, and jm^=m^ . . . , Y m ^ _ i = m ^ , ym^=m^. Thus 1:=^ preserves each m,.
Choose T-stable subalgebras b^, t^ and Ui in m^ where b^=t^@ u^ is a Borel subalge-bra, ti a maximal toral subalgesubalge-bra, etc. Set b^y1"1^ etc. Then the b,, t, and u, are T-stable, b = ® b,, t = © t, and u = ® u, are F-stable, and b, etc. is a Borel subalgebra, etc. ofm.
Given A as in (^), we may write A = A o + A + + A _ where Ao^t, A + e u and A_ e u _ (the opposite nilpotent subalgebra). Then ^ A + and s' A_ are r-invariant, so g (s): = exp (Y A+) exp (^A_) e M (B)^ and g (s) = I + ^r (A^ + A _ ) + 0 (/+1). Hence we may reduce to the case where A e t.
Let oc^, . . .,o^ be the simple roots of m;, and let x{,y{,h{ be corresponding
^-triples, 7=1, . . . , / . Then we may arrange that yx{=x{+^ ^y{=y{^^ 7h{=h{+^
for i<n.
(3.6) Lemma. — Let a=o^ be a simple root ofm^, and let m[ be the subalgebra of mi generated by { ^ x.^y,^ h\k^0] (T=Y"). ^here x=x{, etc. Then m^sl^,
SL, ® ^2-> ^2 ® ^2 ® ^2 or
^S-Proof. - From T we get an automorphism T of the Dynkin diagram of m^. The possibilities are:
(1) m^G^, F4, £7, Eg, B^ or C^: Then T is trivial, i.e., ra=a, and we get a copy ofsl^.
(2) m^ ^ D4: Then ra = a, or TOO = P and T? = a, or TOO = (3, T? = y, TY = a where a, P and Y are distinct. So we get one, two or three copies of sl^. (The simple roots involved are all non-adjacent).
(3) nii^D,,, n^5 or A^.i or E^: Then ra=a, or Po^oc where ra^a and a, TOO are not adjacent. So we get one or two copies of d^.
(4) m^A^fe: If Ta^oc, then in the case of the middle two simple roots we have adjacency. Thus we can have m[ =^l^ sl^ or ^ ® sl^. •
Let m'i=Y~1 m\. Then m7: = © m[ is F-stable. If we can prove (^) for m7, then we can prove it for m, since the toral subalgebras of the various m' span a toral subalgebra ofm. Thus we may assume that m^sl^ or m^=^, i.e., that M is a product of SL^'s or SI^'s. We may also easily reduce to the case that F acts faithfully on M, i.e., d is the order of yeAut(M). Let £, denote the standard primitive d-th root of unity, and set m=d/n.
Case A. — Suppose that m^ =^. Let [x^y^h,] denote the sl^-triple ofm^ where 7^=^+1, Y ^ = ^ + i and Y ^ ^ ^ + i for i<n. Since TeAut(mi) preserves t^ and b^, T / Z I = / Z I and y/^=/^, while yx^ is a multiple of x^. Let /^, . . . , / ? „ ; x\,...,x^
y\, . . ..^ be a basis of eigenvectors for the action of y on span [h^}, etc. One easily shows:
(A.I) Each h\ is a sum Y,Cjhj where no Cj is zero, and similarly for the x\ and y\. The eigenvalues of the h\ are the /7-th roots of unity.
(A. 2) No two x\ have the same eigenvalue (else a linear combination violates A . I ) , and similarly for the y\.
(A. 3) For all / and 7, l^i.j^n, the brackets [h'^x'j}, [h\,y^ and [x\,y^ are non-zero, and span {[x^^]} = span {h'^}.
(A. 4) We may choose the h\, etc. so that (a) The eigenvalue of h\ is ^m(i-l\
(b) 2x;=[^,xJ, ; = 1 , . . . , ^ . (c) - 2 ^ = [ ^ , y j , f = l , . . . , ^ .
Let m^ denote the y-eigenspace of m with eigenvalue ^-fe. Since 7 acts on ^(B) sending the generator s to s ° y, the elements of ^ TH^) are r-invariant.
(3.7) Lemma. — Let l^k^n. Then /^em^, where 0<r^d. There are elements x\ e m^ and y'j e m^ such that h'j, = [x\, y^ where Q<a^r, 0^b<d and a + b = r.
Proof. — Note that r is a multiple o f m . It follows from A. 1, A. 2 and A. 3 that hk=[x^y'j\ for some / and 7. By A. 4, keeping i^-j constant modn, we can arrange that x^em^), 0<a^r. Then y'jCm^ where b is as required. •
Proof of (^) in Case A. — Let A, x and y (without primes) be as in 3.7, i.e., [s" x, sby]=sr h where s'' h, s" x and s1' y are r-invariant, r>0. By construction, a>0. If
&>0, then
exp(sax)exp(sby)exp(-sax)exp(-sby)=l+srh+0(sr+l\
and (^) is established. (Recall that we are inside a product of SL^s, so that exp(sax)=l-\-sax\) If 6=0, then we can replace a by a-m and b by 6 +w, as long as a>m=d/n. Hence the only problem is the case a=r=m, b=0. But then we have:
exp (V x) exp (y) exp (- ^ x) exp ( - y) = I + s' h + ^ (yxy) + 0 (Y +1), where yxy is strictly lower triangular and in m^. Multiplying by exp (-s^xy) we obtain the required element of M (B)^ •
Case B. - Suppose that m^=^. First consider the case n=\, so that m=^
and T = y has even order d=2m. Let a, (3 and oc+ P be the positive roots. Then ra= P, Tp=oc and T ( o c + P ) = o c + P . If x^, j^, /^ and Xp, ^p, Ap are the usual sl^-triples in 513, then T/^=/?p, TX^ is a multiple of Xp, etc. There are linear combinations /^, x1, x2, y1, y2 of the elements above which are eigenvectors of T such that we have:
(B. 1) h^ e(m=sl3)(o), h~ em^. The h±, x1 and yj are linear combinations of the h^ Ap, etc. with non-zero coefficients (e.g. xl=ax^+bx^ where ab^O).
(B. 2) x1 and x2 correspond to distinct eigenvalues of T, and similarly for y1 and
^
(B.3) [/^..x1], [/^y7], [^y7] are non-zero for all /, 7, and we have span { [x\ y3} ] = span { h + }.
(B.4) 2xl=[/^+,xl], 2.x2=[/r,xlL etc.
We now prove (^) in this case (m=^). Note that h^ presents no problem, as
^^[^a+p^a+p] (W to a constant), and we can use our argument for SL^. As in 3.7, we can find x'em^, y^em^ such that h~ =[x\yj] and k-\-l=m.
Let x denote x1, y denote y3. Consider
g (s): = exp (^ x) exp (^ y) exp (-skx) exp (- ^ y).
Ifk, />0, then g ( ^ ) = I + ^mA - +0 (sm+l). Suppose that ^ = 0 (the case 1=0 is similar).
Then we find that g ( ^ ) = I + ^m/ ^-+ ^mA + 0 ( ^m + l) , where A is a sum of products xpyxq and;?+^^2. All such matrices are strictly upper triangular, so we may "correct"
g(s) as in the SL^ case. This completes our argument in case ^2= 1.
If n>\, we proceed as in the case of a product of SL^'s: Let x\, x2, y[, y\, h^~
be the eigenvectors for T in m^. Let x ^ = yl~lx { , etc. be the corresponding elements of n\i. There are linear combinations x^\ x^, y^\ ^.2), /^.±) of the x^, etc. which are eigenvectors for y and satisfy analogues of the properties B. 1-B. 4 above. One proceeds exactly as before. This completes our proof of Theorem 3.5. •