Conway s group and octonions

Transcription

1 Conway s group and octonions Robert A. Wilson School of Mathematical Sciences, Queen Mary, University of London, Mile End Road, London E 4NS Submitted 7th March 009 Abstract We give a description of the double cover of Conway s group in terms of right multiplications by 3 3 matrices over the octonions. This leads to simple sets of generators for many of the maximal subgroups, including a uniform construction of the Suzuki chain of subgroups. Introduction In [] I showed how to construct a 3-dimensional octonionic Leech lattice, based on Coxeter s non-associative ring of integral octonions [5], which is an algebraic version of the E 8 lattice. The automorphism group of the Leech lattice is Conway s group Co 0, which is a double cover of the sporadic simple group Co (see []). In this paper I show how to write generators for Co 0, and many of its maximal subgroups, in terms of 3 3 octonion matrices (suitably interpreted). We begin by summarising the notation and results of []. The octonions are an 8-dimensional real vector space, with basis {i t : t P L(7)}, where P L(7) = { } F 7 is the projective line of order 7, such that i = and the multiplication is given by the images under the subscript permutations t t + and t t of i 0 i = i i 0 = i 3. The lattice L is defined to be the copy of E 8 whose roots t (±i t) with an odd number of minus signs. Let are ±i t ± i u for t u and s = ( + i i 6 ) and R = L. Writing B = LR we proved in [] that B is a copy of the E 8 lattice whose roots are ±i t and the images under t t + of (± ± i 0 ± i ± i 3 ) and (±i ± i 4 ± i 5 ± i 6 ). Thus Coxeter s non-associative ring of integral octonions (see [5] and [3]) is ( + i 0)B( + i 0 ), which is closed under multiplication. Moreover, using the Moufang laws we showed that Ls = B and BL = L. The important content of [] is the following definition.

2 Definition The octonionic Leech lattice Λ = Λ O is the set of triples (x, y, z) of octonions, with the norm N(x, y, z) = (xx + yy + zz), such that (i) x, y, z L, (ii) x + y, x + z, y + z Ls, and (iii) x + y + z Ls. The main result of [] is that Λ is isometric to the Leech lattice. The monomial subgroup The reflection in any vector r of norm in the octonions can be expressed as the map x rxr. In particular, since + i t is perpendicular to s we have s = ( + i t)s( + i t ). Using R a to denote the map x xa, the Moufang law ((xa)b)a = x(aba) can be expressed as R a R b R a = R aba. In particular R s = R +i t R s R +it, which is equivalent to each of the following: Combining two such relations gives Therefore R s R +it = R +it R s, R +it R s = R s R +it. R s R i0 R +it = R +i0 R s R +it = R +i0 R it R s. (L( i 0 ))( + i t ) = (LR)L = BL = L (B( i 0 ))( + i t ) = (BL)R = LR = B ((Ls)( i 0 ))( + i t ) = ((L( + i 0 ))( i t ))s = Ls. These three equations imply that the map R i 0 R +it acting simultaneously on all three coordinates preserves the octonion Leech lattice Λ. Observe that the roots i t for t = 0,,, 3, 4, 5, 6 form a copy of the root system of type A 7, whose Weyl group is the symmetric group S 8. Now the product of the reflections in i 0 and i t is the map x 4 ( i t)(( + i 0 )x( + i 0 ))( i t ) which can be expressed as B +i 0 B i t, where B r denotes the bi-multiplication map x rxr. These elements act as 3-cycles (, 0, t), so generate the rotation

3 subgroup A 8 of the Weyl group. Finally we apply the triality automorphism which takes bimultiplications B u by units u of norm to right-multiplications R u by the octonion conjugate u, and deduce that the maps R i 0 R +it generate. A 8, the double cover of A 8. Adjoining the coordinate permutations and sign changes to this group gives a group. A 8 S 4. Adjoining also the symmetry r 0 : (x, y, z) (x, yi 0, zi 0 ) described in [] gives a group of shape 3+ (A 8 S 3 ). The latter group is in fact a maximal subgroup of the automorphism group. Co of the Leech lattice (see []), so we only need one more (non-monomial) symmetry to generate the whole of. Co. 3 A complex reflection group In order to construct a non-monomial symmetry, we regard s as a complex number ( + 7) and consider the subset of the octonionic Leech lattice which lies inside the 3-dimensional vector space over Q(s) = Q( 7). This is a lattice spanned over Z[s] by the vectors (±s, 0, 0), (±, ±, 0) and (±s, ±s, ±s). Dividing through by s we obtain a well-known lattice (see for example [4, p. 3]) which has 4 vectors of norm 4, and the reflections in these vectors generate the automorphism group of the lattice, which is isomorphic to L 3 (). More explicitly, this automorphism group is generated by the monomial subgroup 3 :S 3 = S4 together with the matrix 0 s s s s which is the negative of reflection in (s,, ). Now this matrix represents the map, (x, y, z) ((y + z)s, xs y + z, xs + y z) () on the given complex vector space. But this can also be interpreted as a map on triples (x, y, z) of octonions. We show next that, with this interpretation, it is also a symmetry of the octonion Leech lattice. To prove this claim, it is useful first to prove the following lemma. Lemma (x, y, z) Λ if and only if the following three conditions hold: (i) x L; (ii) x + y Ls = B; (iii) xs + y + z L. 3

4 Proof. We use repeatedly the properties L Ls L and L Ls L. Suppose the three conditions of the lemma hold. Then y = (x + y) x L so z = (xs + y + z) y xs L. Also y + z = (xs + y + z) xs Ls, and therefore x+z = (x+y) (y +z)+z Ls. Finally x+y +z = (xs+y +z)+xs+x Ls. Conversely if (x, y, z) satisfies the conditions of the original definition, then xs + y + z = (x + y + z)s + (y + z)s + (y + z) L. For convenience, write (x, y, z ) for the image of (x, y, z) under the octonion map given by (), and note that s = ( + 7) satisfies s + s + = 0, so that s + s =, s = s = s and s = s = s. Now we compute (i) x = (y + z)s L; (ii) x + y = (xs + y( + s) + z( + s)) = (x + ys z)s Ls, using the fact that Λ is invariant under coordinate permutations and sign-changes; (iii) x s + y + z = (y + z) + xs L; and the claim is proved. We summarise our results in the following theorem. Theorem The full automorphism group. Co of the Leech lattice is generated by the following symmetries: (i) an S 3 of coordinate permutations; (ii) the map r 0 : (x, y, z) (x, yi 0, zi 0 ); (iii) the maps R i 0 R +it for t =,, 3, 4, 5, 6; (iv) the map (x, y, z) ((y + z)s, xs y + z, xs + y z). 4 The normaliser of the complex reflection group We have described the subgroup. A 8 generated by the maps R i 0 R +it as the double cover of the group of even permutations of {, 0,,, 3, 4, 5, 6}. The stabiliser of is a subgroup. A 7 generated by the elements R i i 0 R it i0 for t =, 3, 4, 5, 6. Since the roots i t i 0 are perpendicular to s, we know that (i t i 0 )s(i t i 0 ) = s and therefore R it i0 R s = R s R it i0. It follows that this group. A 7 commutes with the reflection group L 3 () just described, giving rise to a subgroup L 3 (). A 7. It is well-known (see []) that this group has index in a maximal subgroup of shape (L 3 (). A 7 ).. To obtain the latter group we may adjoin an element such as R i 0 i Rs, where Rs = R s s s. s 4

5 Using the fact that R i i 0 R s = R s R i i 0 we can re-write this element in various ways such as R sr i0 i s s = s s R sr i0 i. s s In particular, it squares to minus the identity. We still have to show that this element is a symmetry of the octonionic Leech lattice. Writing (x, y, z ) for the image of (x, y, z) under this map, we have (i) x = ((xs + y + z)s)(i 4 0 i ) (Ls)L = (LR)L = BL = L and by 4 symmetry also y, z L; (ii) s s = s(s ) = ss = s and therefore x y = ((x y)(i 0 i ))s ((LR)L)s = Ls and again by symmetry y z Ls; (iii) s+ = s and so x +y +z = 4 (((x+y +z)s)(i i 0 ))s (LR)s = Ls. Since, as remarked in [], we can change signs arbitrarily in the definition, we have shown that the given element is a symmetry of the lattice. Finally let us consider the action of R i 0 i Rs by conjugation on L 3 (). A 7. Since the factor. A 7 commutes with the action of any matrix over Q(s), it follows easily that our element acts on the. A 7 factor as the transposition (0, ). Similarly, R i0 i acts as complex conjugation (s s) so our element acts on the L 3 () factor as complex conjugation followed by conjugation by the matrix s s. More concretely, it commutes with the S 3 of coordinate s permutations and maps the sign change on the last two coordinates (that is, the negative of reflection in (, 0, 0)) to the negative of reflection in (s,, ). 5 The Suzuki chain The so-called Suzuki chain of subgroups of. Co is a series of subgroups of the following shapes:. A 9 S 3. A 8 S 4 (. A 7 L 3 ()). (. A 6 U 3 (3)). (. A 5. J ). (. A 4. G (4)). 6. Suz. 5

6 We have already described two groups in this list, namely. A 8 S 4 and (. A 7 L 3 ()).. To obtain. A 9 S 3, we take the S 3 of coordinate permutations, together with the group. A 8 which is generated by R i 0 R +it, and extend. A 7 to. S 7 by adjoining the element R i 0 i Rs as above. The map onto A 9 permuting the points {,, 0,,, 3, 4, 5, 6} is then given by mapping R i0 R +it onto the 3-cycle (, 0, t), as we have already seen, and mapping the extra element to (, )(0, ). In terms of the root system of L, the factor Rs of the new element corresponds to the root s, and extends the root system of type A 7 spanned by i t to one of type A 8. To obtain the remaining groups in the Suzuki chain, all we have to do is adjoin to the complex reflection group L 3 () the part of. A 9 which commutes with the appropriate subgroup. A n. Consider first the subgroup. A 6 generated by R i i R i i t for t = 3, 4, 5, 6. This subgroup centralizes R i 0 Rs as well as the complex reflection group L 3 (). Together these generate the full centralizer U 3 (3), and by adjoining R i 0 R i i we obtain the whole group (. A 6 U 3 (3))., which is a maximal subgroup of. Co. An alternative generating set may be obtained by observing that the monomial subgroup of U 3 (3) is 4 :S 3 generated by r 0 : (x, y, z) (x, yi 0, zi 0 ) and the coordinate permutations. Then we need only adjoin the element () to obtain U 3 (3). To prove that the group is indeed U 3 (3), first observe that all the generating matrices are written over Q(i 0, s), which is an associative ring of quaternions. Therefore these matrices define a quaternionic representation of the group in the usual sense, and it then suffices to reduce the representation modulo 3. Since both i 0 and s s = 7 map to ±i in the field F 9 = F 3 (i), it follows immediately that the generators map to unitary matrices. Notice that this is essentially the same description of the group U 3 (3) as Cohen s description of it as a quaternionic reflection group []. The reflecting vectors are, up to left quaternion scalar multiplication, the = 63 images under the monomial group of the Leech lattice vectors (s, 0, 0), (,, 0) and (s, s, s). The next case may be obtained by taking the subgroup. A 5 generated by R i i 3 R i i t for t = 4, 5, 6. To extend from U 3 (3) to. J we may adjoin R i 0 R +i, or alternatively extend the monomial subgroup from 4 :S 3 to 3+4 :S 3 by adjoining r : (x, y, z) (x, yi, zi ). Then the involution centralizer +4 A 5 is generated by the diagonal symmetries, the coordinate permutation (, 3), and the element R i 0 R s, where R s = s This is reminiscent of, but rather different from, the description of. J as a quaternionic reflection group in [, 7, 8]. The group. (A 4 G (4)). is perhaps best described by taking the subgroup 6

7 . A 4 generated by R i 0 i 3 R i5 i 6 and R i 3 i 5 R i5 i 6. With the quaternionic labelling k i j given in [0] of each brick of the MOG (itself originally described in [6]), these elements become left-multiplication by k and ω respectively. As generators for. G (4) we may take those given above for L 3 (), together with the monomial elements diag(, i t, i t ) for t =,, 4, and the elements R i R i i 4 (equivalent to right multiplication by k) and R i i R i i 4 (equivalent to right multiplication by ω), as well as R i Rs. The last case 6. Suz is of particular interest. It may be taken as the centralizer of the element R i 3 i 5 R i5 i 6 of order 6. Then it is generated by the sign-changes and coordinate permutations, together with the matrix of reflection in (s,, ), and R i 0 R +it for t =,, 4, as well as R i Rs. This extends to 6. Suz: by adjoining R i 0 R i5 i 6. 6 Some -local subgroups We have already seen how to generate the maximal -local subgroup of Co 0 which has shape 3+ (A 8 S 3 ), acting monomially on the three octonion coordinates. The involution centraliser has the shape +8. W (E 8 ). If we take our involution to be r 0 = diag(,, ), then the normal subgroup +8 is generated by r t = diag(, i t, i t ) for all t. Modulo this, the group. A 8 together with diag(i t, i t, ) generate a maximal subgroup of W (E 8 ), which may be extended to the whole group by adjoining an element such as R i 0 R s, where R s is as defined above. Another maximal -local subgroup of Co 0 is M 4. This intersects our monomial group 3+ (A 8 S 3 ) in a group. 6 (S 3 L 3 ()) = 3+.( 3 L 3 () S 3 ). To make the latter group, take the normal subgroup 3+ S 3 generated by the coordinate permutations and all r t, and the subgroup of A 8 generated by 8 R i R i3 i 5 R i4 i R i6 i 0 R i R i i 4 and its images under i t i t+. Now let π be the coordinate permutation (, ), and adjoin the element r πr i R s/ = 4 R i R sdiag(i,, i )R i R s. 7 j k i

8 A little calculation shows that this element lies in the subgroup M 4 which acts monomially on the MOG described above, but does not lie in 3+ (A 8 S 3 ). Hence we have obtained generators for M 4. A similar process gives generators for the other -constrained maximal - local subgroup 5+ (S 3 3S 6 ). In this case we adjoin the above element to the subgroup 3+ ((A 4 A 4 ). S 3 ) obtained by restricting to the subgroup of. A 8 generated by R i R +i, R i R +i4, R i 0 i 3 R i0 i 6, R i 0 i 3 R i0 i 5 and R 4 i 0 R i i 3 R i i 6 R i4 i 5. References [] A. M. Cohen, Finite quaternionic reflection groups, J. Algebra 64 (980), [] J. H. Conway, A group of order 8,35,553,63,086,70,000, Bull. London Math. Soc. (969), [3] J. H. Conway and D. A. Smith, On quaternions and octonions: their geometry, arithmetic and symmetry, A. K. Peters (003). [4] J. H. Conway, R. T. Curtis, S. P. Norton, R. A. Parker and R. A. Wilson, An Atlas of Finite Groups, Clarendon Press, Oxford, 985. [5] H. S. M. Coxeter, Integral Cayley numbers, Duke Math. J. 3 (946), [6] R. T. Curtis, A new combinatorial approach to M 4, Math. Proc. Cambridge Philos. Soc. 79 (976), 5 4. [7] J. Tits, Quaternions over Q( 5), Leech s lattice and the sporadic group of Hall Janko, J. Algebra 63 (980), [8] R. A. Wilson, The geometry of the Hall Janko group as a quaternionic reflection group, Geom. Dedicata 0 (986), [9] R. A. Wilson, The complex Leech lattice and maximal subgroups of the Suzuki group, J. Algebra 84 (983), [0] R. A. Wilson, The quaternionic lattice for G (4) and its maximal subgroups J. Algebra 77 (98), [] R. A. Wilson, The maximal subgroups of Conway s group Co, J. Algebra 85 (983), [] R. A. Wilson, Octonions and the Leech lattice, J. Algebra 3 (009),