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THE INVOLUTION WIDTH of FINITE SIMPLE GROUPS Alex Malcolm · Supervisor: Prof

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THE INVOLUTION WIDTH of FINITE SIMPLE GROUPS Alex Malcolm · Supervisor: Prof THE INVOLUTION WIDTH OF FINITE SIMPLE GROUPS Alex Malcolm · Supervisor: Prof. Martin Liebeck Department of Mathematics, Imperial College London, SW7 2AZ [email protected] WIDTH QUESTIONS MAIN THEOREM Let G be a finite group and let S be a normal subset of G Theorem: (M. 2016, [3]) Every non-abelian finite simple group (i.e. a subset closed under conjugation). Suppose that S has involution width at most 4. generates the group, then for each g 2 G, there exists an expression g = t1 ::: tk where ti 2 S. It follows that we can GROUPS OF LIE TYPE write G = Sd [ Sd−1 [···[ S [ f1g and we define the width The general procedure for finding the involution width of a of G with respect to S to be the minimal d for which this simple group of lie type G, is as follows. We aim to pick holds. We denote this by width¹G; Sº. Examples: regular semisimple elements x and y in particular classes of G, such that I S = fI¹Gºg, where I¹Gº is the set of involutions. So width¹G; I¹Gºº is the minimal n such that every element I ¹1º The elements x and y are strongly real. is the product of at most n involutions. I ¹2º Gnf1g ⊂ xG · yG: I S = fC¹Gºg, where C¹Gº = f»x; y¼ : x; y 2 Gg - the commutator width From ¹1º and ¹2º it follows that width¹Gº ≤ k width¹xº+width¹yº ≤ 4. For ¹2º it suffices to show that I S = fPk¹Gºg, where k ≥ 2 and Pk¹Gº = fx : x 2 Gg is the set of kth powers in G. Õ χ¹xºχ¹yºχ¹g−1º , 0 χ¹1º Width questions are not independent for different choices of χ2Irr¹Gº S e.g. if k is odd then the kth power map fixes every for all 1 , g 2 G ([1]). involution and it follows that I Complete character tables are currently unavailable for width¹G; Pk¹Gºº ≤width¹G; I¹Gºº. most groups of Lie type. We therefore rely on estimates Theorem: ([1]). There is an absolute constant c > 0 such of character values derived from the Deligne-Lusztig that for any finite non-abelian simple group G and any representation theory. n non-identity normal subset S ⊆ G , we have G = S for all I Substantial difficulties are faced in the case of unitary n ≥ c logjGj/logjSj. groups, where we develop the theory of minimal degree characters using dual pairs. Corollary: There is an absolute constant N such that every Similarly problematic are a number of exceptional groups element of every finite non-abelian simple group is a product I of Lie type. Here we use an inductive approach, of N involutions. restricting to subgroups of G for which the involution AIM: Find the minimal value for N. width is known STRONGLY REAL SIMPLE GROUPS ALTERNATING AND SPORADIC GROUPS Def: An element x 2 G is real if xg = x−1 for some g 2 G. x For these groups we can say more, and consider the width is strongly real if g can be taken to be an involution. The with regards to elements of any fixed prime order. We call group G is (strongly) real if all elements are (strongly) real. this the p-width of the group. Lemma: x 2 G is strongly real if and only if x is a product of Theorem: (M. 16). Fix a prime p. The p width of An ¹n ≥ pº 2 involutions. is at most 3. Naturally if G is strongly real then we have a width as small Theorem: (M. 16). Let ¹G; pº be a pair consisting of a as possible. The real FSGs were classified by Tiep and sporadic finite simple group G and a prime p, dividing jGj. Zalesski [4] and it was later shown that G is real if and only if Then the p-width of G is 2, unless it is listed below (the strongly real. This was completed by a series of different p-width is 3 in these cases). authors working through each family of the classification. Compiling the results we have Prime Width 3 Groups Theorem: A finite simple group is strongly real if and only if 2 All except J1 and J2 it is one of the following 3 HS, Co2, Co3, Fi22, Fi23, BM I PSp2n¹qº where q . 3 ( mod 4 ) and n ≥ 1; 5 Fi22, Fi23 I PΩ2n+1¹qº where q ≡ 1 ( mod 4 ) and n ≥ 3; I PΩ9¹qº where q ≡ 3 ( mod 4 ); + ¹ º ≥ I PΩ4n q where q . 3 ( mod 4 ) and n 3; REFERENCES − ¹ º ≥ [1] Z. Arad and M., Herzog (eds.), Products of Conjugacy Classes in Groups. Lecture Notes in Math. I PΩ4n q n 2; +¹ º 3 ¹ º 1112, Springer-Verlag, Berlin (1985). I PΩ8 q or D4 q ; [2] M. W. Liebeck and A. Shalev, Diameters of finite simple groups: sharp bounds and applications. Ann. A ; A ; A ; A , J , J . of Math. 154 (2001), 383–406. I 5 6 10 14 1 2 [3] A. J. Malcolm, The involution width of finite simple groups. arXiv:1611.06900 (2016) [4] P. H. Tiep and A. E. Zalesskii, Real conjugacy classes in algebraic groups and finite groups of Lie type, J. Group Theory 8 (2005), 291-315..
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