A2-proof of Structure Theorems for Chevalley Groups of Type

A new geometric proof is given for the standard description of subgroups in the Chevalley group G = G(F4, R) of type F4 over a commutative ring R that are normalized by the elementary subgroup E(F4, R). There are two major approaches to the proof of such results. Localization proofs (Quillen, Suslin, Bak) are based on a reduction in the dimension. The first proofs of this type for exceptional groups were given by Abe, Suzuki, Taddei and Vaserstein, but they invoked the Chevalley simplicity theorem and reduction modulo the radical. At about the same time, the first author, Stepanov, and Plotkin developed a geometric approach, decomposition of unipotents, based on reduction in the rank of the group. This approach combines the methods introduced in the theory of classical groups by Wilson, Golubchik, and Suslin with ideas of Matsumoto and Stein coming from representation theory and Ktheory. For classical groups in vector representations, the resulting proofs are quite straightforward, but their generalizations to exceptional groups require an explicit knowledge of the signs of action constants, and of equations satisfied by the orbit of the highest weight vector. They depend on the presence of high rank subgroups of types Al or Dl, such as A5 ≤ E6 and A7 ≤ E7. The first author and Gavrilovich introduced a new twist to the method of decomposition of unipotents, which made it possible to give an entirely elementary geometric proof (the proof from the Book) for Chevalley groups of types Φ = E6,E7. This new proof, like the proofs for classical cases, relies upon the embedding of A2. Unlike all previous proofs, neither results pertaining to the field case nor an explicit knowledge of structure constants and defining equations is ever used. In the present paper we show that, with some additional effort, we can make this proof work also for the case of Φ = F4. Moreover, we establish some new facts about Chevalley groups of type F4 and their 27-dimensional representation. In the present paper, which is a sequel to the work by the first author and Mikhail Gavrilovich [12], we prove the main structure theorems for the Chevalley group G = G(F4, R) of type F4 over an arbitrary commutative ring R. More precisely, we describe subgroups of G(F4, R) normalized by the elementary subgroup E(F4, R). As such, this result is not new, since structure theorems are known for all Chevalley groups. For classical groups they were established in [38, 45, 82, 18, 19, 20, 3, 55, 56, 70, 72, 73, 74, 48, 49, 27, 66, 43], for exceptional ones — in [30, 31, 32, 34, 67, 68, 69, 71, 50]; see also further references in [12, 13, 42, 43, 66, 75, 80]. Thus, the gist of the present paper, as that of [12], resides not in the results themselves, but rather in the method of their proof. Namely, we propose a new geometric approach towards calculations in Chevalley groups of type F4, which involves as little information about the group as possible. 2000 Mathematics Subject Classification. Primary 20G15, 20G35.