Automorphisms and twisted forms of Lie conformal superalgebras
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1 Algebra Seminar Automorphisms and twisted forms of Lie conformal superalgebras Zhihua Chang University of Alberta April 04, Dept of Math and Stats, University of Alberta, Edmonton, Alberta, T6G 2G1, Canada. 1 / 40
2 2 / 40 Outlines Twisted forms of finite dimensional algebras. Differential conformal superalgebras and their twisted forms. The case of the N = 1, 2, 3 Lie conformal superalgebras. The case of the N = 4 Lie conformal superalgebras.
3 3 / 40 Twisted affine Kac-Moody algebras Let g a finite dimensional simple Lie algebra over k, σ an automorphism of g of order m. The twisted loop algebra L(g, σ) is the subalgebra L(g, σ) = i Z g i k kt i m g k k[t ± 1 m ], where g i = {x g σ(x) = ζ i mx}, ζ m is a m-th primitive root of unit. In particular, L(g, id) = g k k[t ±1 ].
4 4 / 40 Twisted affine Kac-Moody algebras (continued) Facts: Every affine Kac-Moody algebra is an extension of a twisted loop algebra. L(g, σ) is not only a Lie algebra over k, but also a Lie algebra over the ring k[t ±1 ]. L(g, σ) k[t ±1 ] k[t ± 1 m ] = L(g, id) k[t ±1 ] k[t ± 1 m ] = g k k[t ± 1 m ]. L(g, σ) is a twisted form of L(g, id) with respect to the ring extension k[t ±1 ] k[t ± 1 m ].
5 Twisted forms of algebras Setting: R is a commutative ring. A is an algebra over R. R S a ring homomorphism. Change of base ring from R to S: An S algebra structure on A R S given by (a r)(b s) = ab rs, a, b A, r, s S S/R-form of A: An R algebra B is called a S/R form of A if A R S = B R S as S algebras. 5 / 40
6 6 / 40 Classification of twisted forms Theorem Let A be an algebra over R and R S a faithfully flat ring extension, then the set of isomorphism classes of S/R forms of A one to one H 1 (S/R, Aut(A)).
7 7 / 40 The automorphism group functor Aut(A): a functor from the category of R algebras to the category of groups S Aut S (A R S). For a finite dimensional algebra A over k, Aut(A ) is a linear algebraic group. Aut(A ) is a representable functor: R Aut R (A k R). If A = A k R, then Aut(A) = Aut(A ) R.
8 8 / 40 Outlines Twisted forms of finite dimensional algebras. Differential conformal superalgebras and their twisted forms. The case of the N = 1, 2, 3 Lie conformal superalgebras. The case of the N = 4 Lie conformal superalgebras.
9 9 / 40 Lie conformal algebras over k Definition A k Lie conformal algebra is k[ ] module A equipped with a k bilinear product (n) : A A A for each n N satisfying: for a, b, c A, (C0) a (n) b = 0 for n 0, (C1) A (a) (n) b = na (n 1) b, (C2) a (n) b = j N ( 1)j+n (j) A (b (n+j)a), (C3) a (m) (b (n) c) = m ( m ) j=0 j (a(j) b) (m+n j) c + b (n) (a (m) c),
10 10 / 40 The λ bracket Let A be a k Lie conformal algebra. For a, b A, define where λ (n) = 1 n! λn. [a λ b] = n N λ (n) a (n) b, The axioms (C0)-(C3) are equivalent respectively to (C0) λ [a λ b] is a polynomial in λ, (C1) λ [( a) λ b] = λ[a λ b], (C2) λ [a λ b] = [b λ a], (C3) λ [a λ [b µ c]] = [[a λ b] λ+µ c] + [b µ [a λ c]].
11 11 / 40 Example The centreless Virasoro Lie algebra: basis: {l n, n Z}, [l m, l n ] = (m n)l m+n, m, n Z. Form a formal series: L(z) = n Z l n z n 2. The operator product expansion: [L(z), L(w)] := [l m, l n ]z m 2 w n 2 where m,n Z = ( w L(w))δ(z w) + 2L(w) w δ(z w) δ(z w) = 1 z n Z ( ) w n. z
12 12 / 40 Example (continued) The Virasoro Conformal algebra: the k[ ] module V = k[ ]L. the n-th product: L (0) L = L, L (1) L = 2L, L (n) L = 0, n 2. The λ bracket [L λ L] = ( + 2λ)L.
13 Differential rings To define conformal superalgebra over a ring, rings = differential rings A k differential ring is a pair R = (R, δ) consisting of a commutative associative k algebra R, and a k linear derivation δ : R R. k differential rings form a category k-drng, where a morphism f : R = (R, δ R ) S = (S, δ S ) is a homomorphism of k algebra f : R S such that f δ R = δ S f. 13 / 40
14 14 / 40 Differential Lie conformal superalgebras Definition Let R = (R, δ) be a k differential ring, an R Lie conformal superalgebra is a triple (A, A, ( (n) ) n N ) consisting of a Z/2Z graded R module A = A 0 A 1, A End k (A) stabilizing the even and odd parts of A, a k bilinear product (a, b) a (n) b, a, b A for each n N, satisfying the following axioms for r D, a, b, c A, m, n N: (CS0) a (n) b = 0 for n 0, (CS1) A (a) (n) b = na (n 1) b and a (n) A (b) = A (a (n) b) + na (n 1) b, (CS2) A (ra) = r A (a) + δ(r)a, (CS3) a (n) (rb) = r(a (n) b) and (ra) (n) b = j N δ(j) (r)(a (n+j) b), (CS4) a (n) b = p(a, b) j N ( 1)j+n (j) A (b (n+j)a), (CS5) a (m) (b (n) c) = m j=0 ( m j ) (a(j) b) (m+n j) c + p(a, b)b (n) (a (m) c),
15 Change the base ring Let R = (R, δ R ) be a k differential ring, A an R-Lie conformal superalgebra, R = (R, δ R ) S = (S, δ S ) an extension of k differential rings. Then we can form a S Lie conformal superalgebra A R S: The underlying S module: A R S, A R S = A id + id δ S, The n-th product for n N: (a f ) (n) (b g) = j N a (n+j) b δ (j) S (f )g, where δ (j) S = 1 j! δj S. 15 / 40
16 16 / 40 Twisted forms Definition (Kac, Lau, Pianzola, 2009) Let R S be an extension of k differential rings, A an R Lie conformal superalgebra. An S/R form of A is an R Lie conformal superalgebra B such that B R S = A R S as S Lie conformal superalgebras.
17 17 / 40 Automorphisms Let A be an R Lie conformal superalgebra, a map φ : A A is called an R automorphism of A if: φ is an isomorphism of Z/2Z graded R modules, φ A = A φ, φ preserves all n-th products. All R automorphisms of A form a group, denoted by Aut R-conf (A).
18 18 / 40 Automorphism group functor The automorphism group functor Aut(A): the category of R extensions the category of groups S Aut S-conf (A R S)
19 19 / 40 Classification of twisted forms Theorem (Kac, Lau, Pianzola, 2009) Let R S be a faithfully flat extension of k differential rings, A an R conformal superalgebra. Then the set of isomorphism classes of S/R forms of A (up to R automorphism) one to one H 1 (S/R, Aut(A))
20 20 / 40 Twisted loop conformal superalgebras Let A be a Lie conformal superalgebra over k, σ an automorphism of A of order m, D = (k[t ±1 ], d dt ), D m = (k[t ± 1 m ], d dt ), D = lim D m = (k[t q, q Q], d dt ). A twisted loop Lie conformal algebra L(A, σ) = i Z A i kt i m A k D m where A i = {a A σ(a) = ζ i ma}, i Z, ζ m is a primitive m-th root of unit.
21 21 / 40 Twisted loop conformal superalgebras (continued) Facts: L(A, σ) is a D Lie conformal superalgebra; L(A, σ) is a D m /D form of L(A, id) = A k D. L(A, σ) is a D/D form of L(A, id).
22 Finiteness condition Proposition Let A an D conformal superalgebras. If A satisfies: There exist a 1,, a n A such that { l A (ra i) r D, l 0} spans A. Then H 1 ( D/D, Aut(A)) = H 1 ct(π 1 (D), Aut(A)( D)), where π 1 (D) is the algebraic fundamental group of Spec(D) at the geometric point a = Spec(C(t)) and H 1 ct denotes the continuous non-abelian cohomology of the profinite group π 1 (D) = Ẑ acting (continuously) on the group Aut(A)( D). 22 / 40
23 23 / 40 Centroid trick method The centroid of A is Ctd R (A) = {χ End R-smod (A) χ(a (n) b) = a (n) χ(b), a, b A, n Z + }, Proposition Let A i, i = 1, 2 be two R conformal superalgebras. Assume that Aut k (R) = 1. If R Ctd k (A i ), i = 1, 2 are both k algebra isomorphisms. Then A 1 = A2 as k Lie conformal superalgebras if and only if A 1 = A2 as R Lie conformal superalgebras.
24 24 / 40 Outlines for completed works Automorphisms and twisted forms of N = 1, 2, 3 Lie conformal superalgebras Automorphisms of the N = 4 Lie conformal superalgebras
25 25 / 40 N = 1, 2, 3 Lie conformal superalgebras K N K N = k[ ] k Λ(N), where Λ(N) is the Grassmann superalgebra over k in N variables ξ 1,, ξ N. For n N, the n-th product on K N is given by ( ) 1 f (0) g = 2 f 1 fg ( 1) f ( ) 1 f (1) g = ( f + g ) 2 fg, 2 f (n) g = 0, n 2. N i=1 ( i f )( i g)
26 26 / 40 A subgroup functor GrAut(K N )(R) = {φ Aut(K N )(R) φ(k k Λ(N) k R) k k Λ(N) k R}. Theorem Let R = (R, δ) be an arbitrary k differential ring. For N = 1, 2, 3, GrAut(K N )(R) = O N (R), where O N is the group scheme of N N orthogonal matrices. The isomorphism is functorial in R.
27 27 / 40 The construction of the isomorphism. Given A O N (R), it defines an automorphism ϕ A of K N k R N = 1, A = a R, ϕ A (1) = 1, ϕ A (ξ 1 ) = ξ 1 a. N = 2, A = (a ij ), φ A (1) = 1 + ξ 1 ξ 2 r, φ A (ξ 1 ) = ξ 1 a 11 + ξ 2 a 21, φ A (ξ 1 ξ 2 ) = ξ 1 ξ 2 det(a), φ A (ξ 2 ) = ξ 1 a 12 + ξ 2 a 22, where ( ) 0 r = 2δ(A)A r 0 T.
28 28 / 40 The construction of the isomorphism (continued) For N = 3, 3 φ A (1) = 1 + ɛ mnl ξ m ξ n r l, φ A (ξ j ) = l=1 3 ξ l a lj + ξ 1 ξ 2 ξ 3 s j, l=1 φ A (ξ i ξ j ) = ɛ ijl 3 ɛ mnl ξ m ξ n A l l, l =1 φ A (ξ 1 ξ 2 ξ 3 ) = ξ 1 ξ 2 ξ 3 det(a), i, j = 1, 2, 3, i j, where A l l is the cofactor of a l l in A and 0 r 3 r 2 r 3 0 r 1 = 2δ(A)A T, 0 s 3 s 2 s 3 0 s 1 = 2(det A)A T δ(a). r 2 r 1 0 s 2 s 1 0
29 29 / 40 Theorem Let R = (R, δ) be a k differential ring. If R is an integral domain, then for N = 1, 2, 3, GrAut(K N )(R) = Aut(K N )(R).
30 30 / 40 Classification up to R isomorphism Theorem Let N = 1, 2, 3. There are exactly two D/D forms of K N k D (up to isomorphism of D Lie conformal superalgebras): L(K N, id) and L(K N, ω N ).
31 31 / 40 Classification up to k isomorphism Proposition (Chang, Pianzola, 2011) Let A = L(K N, σ) where σ is an automorphism of K N of order m, and N = 1, 2, 3. Then the canonical map D Ctd k (A) is a k algebra isomorphism. Theorem (Chang, Pianzola, 2011) There are exactly two twisted loop Lie conformal superalgebra (up to k isomorphism) based on each K N, N = 1, 2, 3: L(K N, id) and L(K N, ω N ).
32 32 / 40 Corresponding Lie superalgebras Let A be a k Lie conformal superalgebra, every twisted loop algebra L(A, σ) corresponds to a Lie superalgebra over k: L(A, σ) Alg(A, σ) = ( ), + d dt L(A, σ) where the super bracket comes from the 0-th product on L(A, σ). For K N, N = 1, 2, 3, L(K N, id) and L(K N, ω N ) yield Alg(K N, id) and Alg(K N, ω N ). Theorem (Chang, Pianzola, 2011) Alg(K N, id) and Alg(K N, ω N ) are non-isomorphic.
33 33 / 40 Outlines for completed works Automorphisms and twisted forms of N = 1, 2, 3 Lie conformal superalgebras Automorphisms of the N = 4 Lie conformal superalgebras
34 34 / 40 N = 4 Lie conformal superalgebra F where F = F 0 F 1 = (k[ ] V 0) (k[ ] V 1), V 0 = kl kt 1 kt 2 kt 3, V 1 = kg 1 kg 2 kg 1 kg 2. The λ bracket on L is given by [L λ L] = ( + 2λ)L, [L λ T i ] = ( + λ)t i, [L λ G p ] = ( λ) G p, [T i λt j ] = iɛ ijk T k, [ Lλ G p ] ( = λ) G p, [G p λg q ] = 0, [T i λg p ] = q=1 σi pqg q [ p, G λg q ] = 0, [ T i λ G p ] = q=1 σi qpg q, [ G p λ G q ] = 2δpq L 2( + 2λ) 3 i=1 σi pqt i,
35 35 / 40 A subgroup functor GrAut(F)(R) = {φ Aut(F)(R) φ(k k V k R) k k V k R}, where V = V 0 V 1 and R = (R, δ) is a k differential ring.
36 36 / 40 GrAut(F) Theorem For every k differential ring R = (R, δ), there is an exact sequence of groups: 1 µ 2 (R 0 ) SL 2 (R) SL 2 (R 0 ) ιr GrAut(F)(R), where R 0 = ker δ, SL 2 is the group scheme of matrices with determinant 1, µ 2 is the group scheme of square roots of 1. The exact sequence is functorial in R.
37 37 / 40 The surjectivity of ι R Proposition For every k differential ring R = (R, δ R ) with R an integral domain, and ϕ GrAut(F)(R), there is an étale extension S = (S, δ S ) of R, and an element (A, B) SL 2 (S) SL 2 (S 0 ) such that ι S (A, B) = ϕ S, where ϕ S is the image of ϕ under GrAut(F)(R) GrAut(F)(S). Proposition Let R = (R, δ) be a k differential ring. If R is an integral domain, and the étale cohomology set H 1 ét (R, µ 2) is trivial, then ι R is surjective.
38 38 / 40 Aut(F) Theorem Let R = (R, δ) be a k differential ring such that R is an integral domain. Then GrAut(F)(R) = Aut(F)(R).
39 39 / 40 References Z. Chang, and A. Pianzola, Automorphisms and twisted forms of the N = 1, 2, 3 Lie conformal superalgebras, Communications in Number Theory and Physics, 5(4), , (2011). V. Kac, M. Lau and A. Pianzola, Differential Conformal superalgebras and their forms, Advances in Mathematics 222 (2009), A. Schwimmer, N. Seiberg, Comments on the N = 2, 3, 4 superconformal algebras in two dimensions, Physics Letters B, 184, , (1987). W. Waterhouse, Introduction to Affine Group Schemes, Graduate Texts in Mathematics 66, Springer, (1979).
40 40 / 40 Thank You!
University of Alberta AUTOMORPHISMS AND TWISTED FORMS OF DIFFERENTIAL LIE CONFORMAL SUPERALGEBRAS. Zhihua Chang. Doctor of Philosophy.
University of Alberta AUTOMORPHISMS AND TWISTED FORMS OF DIFFERENTIAL LIE CONFORMAL SUPERALGEBRAS by Zhihua Chang A thesis submitted to the Faculty of Graduate Studies and Research in partial fulfillment
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