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Lecture 5: More Constructions of Vertex Algebras The coset and BRST constructions More constructions of vertex algebras The Borcherds bicharacter construction Monstrous Moonshine The orbifold construction Lecture 5: More constructions of vertex algebras Lecture 5 of 8: More constructions of vertex algebras Dominic Joyce, Oxford University Summer term 2021 References for this lecture: Frenkel and Ben-Zvi, 2nd ed. (2004), x5, R. Borcherds, Quantum vertex algebras, math.QA/9903038. These slides available at http://people.maths.ox.ac.uk/∼joyce/ 1 / 39 Dominic Joyce, Oxford University Lecture 5: More constructions of vertex algebras The coset and BRST constructions More constructions of vertex algebras The Borcherds bicharacter construction Monstrous Moonshine The orbifold construction Plan of talk: 5 More constructions of vertex algebras 5.1 The coset and BRST constructions 5.2 The Borcherds bicharacter construction 5.3 The orbifold construction 2 / 39 Dominic Joyce, Oxford University Lecture 5: More constructions of vertex algebras The coset and BRST constructions More constructions of vertex algebras The Borcherds bicharacter construction Monstrous Moonshine The orbifold construction 5.1. The coset and BRST constructions The coset construction Definition 5.1 (Frenkel{Zhu 1992.) Let (V ; 1; ezD ; Y ) be a vertex algebra over R, and W ⊂ V be an R-submodule. Write C(V ; W ) for the R-submodule of V spanned by v 2 V such that Y (w; z)v 2 V [[z]] for all w 2 W . This implies that if v 2 C(V ; W ) and w 2 W then Y (v; z1) ◦ Y (w; z2) = Y (w; z2) ◦ Y (v; z1): Hence if v1; v2 2 C(V ; W ) then Y (v1; z1)◦Y (v2; z2) ◦Y (w; z3)=Y (v1; z1)◦Y (w; z3)◦Y (v2; z2) = Y (w; z3) ◦ Y (v1; z1) ◦ Y (v2; z2) : Using this we deduce that 1 ±1 ±1 Y (v1; z1) ◦ Y (v2; z2) 2 C(V ; W )[[z1 ; z2 ]], so C(V ; W ) is a vertex subalgebra of V , called a coset vertex algebra. 3 / 39 Dominic Joyce, Oxford University Lecture 5: More constructions of vertex algebras The coset and BRST constructions More constructions of vertex algebras The Borcherds bicharacter construction Monstrous Moonshine The orbifold construction Example 5.2 Let g be a reductive Lie algebra over C, with non-degenerate inner product h ; i; and let h ⊂ g be a reductive Lie subalgebra, with h ; ijh×h nondegenerate. Generalizing Example 3.10, we have a nontrivial central extension of Lie algebras defined using h ; i −1 0 / hciC / g^ / g[t; t ] / 0; ^g and for k 2 C we can define an induced representation Vk of g^ and make it into a vertex algebra using the Reconstruction Theorem. For non-critical k this is a VOA with conformal vector !g. ^h ^g There is a natural inclusion of vertex algebras Vk ,! Vk . Then ^g ^h C(Vk ; Vk ) is a coset vertex algebra. For non-critical k it is a VOA with conformal vector !g=h = !g − !h. 4 / 39 Dominic Joyce, Oxford University Lecture 5: More constructions of vertex algebras The coset and BRST constructions More constructions of vertex algebras The Borcherds bicharacter construction Monstrous Moonshine The orbifold construction The BRST construction Lemma 5.3 Let (V ; 1; ezD ; Y ) be a vertex algebra over R. Then for all u; v 2 V we have (k) (k) u0(1)=0; u0(D (v))=D (u0(v)); [u0; Y (v; z)]=Y (u0(v); z): Hence u0 : V ! V is an infinitesimal automorphism of the vertex algebra structure. Definition 5.4 Let (V ; 1; ezD ; Y ) be a vertex algebra over R with an additional compatible -grading V = L V n. Suppose Q 2 V 1 with Z n2Z Q0 ◦ Q0 = 0. Then we can use Lemma 5.3 to show that Ker Q0 is a vertex subalgebra of V , and Im Q0 is an ideal in Ker Q0. Thus the cohomology Ker Q0= Im Q0 of Q0 is a vertex algebra. Also 0 0 (V \ Ker Q0)=(V \ Im Q0) is a vertex subalgebra of this. 5 / 39 Dominic Joyce, Oxford University Lecture 5: More constructions of vertex algebras The coset and BRST constructions More constructions of vertex algebras The Borcherds bicharacter construction Monstrous Moonshine The orbifold construction The BRST construction comes up in several places. In Physics, it is related to quantizing field theories with gauge symmetries. Lian{Zuckerman define topological vertex operator algebras, which are graded VOAs (V ∗; 1; ezD ; Y ) with elements Q; F ; G 2 V ∗ ∗ satisfying some identities. The BRST VOA H (Q0) has the additional structure of a Batalin{Vilkovisky algebra. The BRST construction is used to define W-algebras, an important class of vertex algebras. 6 / 39 Dominic Joyce, Oxford University Lecture 5: More constructions of vertex algebras The coset and BRST constructions More constructions of vertex algebras The Borcherds bicharacter construction Monstrous Moonshine The orbifold construction 5.2. The Borcherds bicharacter construction Definition 5.5 Let K be a field of characteristic zero. Let (V ; ·; ∆; 1; ) be a commutative, cocommutative bialgebra over K, with product · : V × V ! V , coproduct ∆ : V ! V ⊗ V , unit 1 and counit P 0 00 : V ! K. We use the notation ∆v = v v ⊗ v , which is short P 0 00 0 00 for ∆v = i2I vi ⊗ vi for some finite set I and vi ; vi 2 V . Let D : V ! V be a derivation of (V ; ·; ∆; 1; ). That is, D is K-linear with D(u · v) = (Du) · v + u · (Dv) and P 0 00 0 00 D ◦ ∆v = v (D(v ) ⊗ v + v ⊗ D(v )). A bicharacter for (V ; ·; ∆; 1; ); D is a K-bilinear map −1 rz : V × V ! K[z; z ] satisfying: (i) rz (u; v) = r−z (v; u); (ii) rz (u; 1) = (u); P 0 00 (iii) rz (u · v; w) = w rz (u; w )rz (v; w ); and d (iv) rz (u; Dv) = −rz (Du; v) = dz rz (u; v). 7 / 39 Dominic Joyce, Oxford University Lecture 5: More constructions of vertex algebras The coset and BRST constructions More constructions of vertex algebras The Borcherds bicharacter construction Monstrous Moonshine The orbifold construction Definition 5.5 (Continued.) Define Y : V ⊗ V ! V [[z]][z−1] by X zn X X Y (u; z)v = r (u0; v 0)(Dnu00) · v 00: (5.1) n! z n>0 u v Theorem 5.6 (Borcherds 1999.) zD In Definition 5.5, (V ; 1; e ; Y ) is a vertex algebra over K. There are also super- and graded versions: we take (V∗; ·; ∆; 1; ) to be super- or graded (co)commutative, and D graded of degree −1 2, and r : V × V ! K[z; z ] to be graded with deg z = −2. 8 / 39 Dominic Joyce, Oxford University Lecture 5: More constructions of vertex algebras The coset and BRST constructions More constructions of vertex algebras The Borcherds bicharacter construction Monstrous Moonshine The orbifold construction Sketch proof of Theorem 5.6 P 0 00 000 Let u; v; w 2 V . Write (∆ × idV ) ◦ ∆u = u u ⊗ u ⊗ u , and similarly for v; w. Consider the element of the meromorphic −1 −1 −1 function space V [[z1; z2]][z1 ; z2 ; (z1 − z2) ]: X zmzn X X X 1 2 r (u0; w 0)r (v 0; w 00)r (u00; v 00) (5.2) m!n! z1 z2 z1−z2 m;n>0 u v w (Dmu000) · (Dnv 000) · w 00: (This is X3(z1; z2; 0)(u ⊗ v ⊗ w) in the notation of x2.3.) We can prove that all of Y (u; z1) ◦ Y (v; z2)w; Y (v; z2) ◦ Y (u; z1)w; Y (Y (u; z1 − z2)v; z2)w are power series expansions of (5.2) using iz1;z2 ; iz2;z1 ; iz1−z2;z2 . Thus we can deduce that Y satisfies weak commutativity and weak associativity, the main vertex algebra axioms. 9 / 39 Dominic Joyce, Oxford University Lecture 5: More constructions of vertex algebras The coset and BRST constructions More constructions of vertex algebras The Borcherds bicharacter construction Monstrous Moonshine The orbifold construction The bicharacter construction is a good place to start for thinking about generalizations of vertex algebras. For example, if we omit the condition that rz (u; v) = r−z (v; u) we get a noncommutative generalization of vertex algebras called nonlocal vertex algebras or field algebras. There are also `deformed' notions of vertex algebra in which rz1;z2 depends on two variables z1; z2. Example 5.7 −1 For any (V ; ·; ∆; 1; ); D, define rz : V × V ! K ⊂ K[z; z ] by rz (u; v) = (u)(v). This is a bicharacter (the trivial bicharacter), and (V ; 1; ezD ; Y ) is the commutative vertex algebra defined from (V ; ·; 1); D in x1.3, and is independent of the coproduct ∆. In general we can think of (V ; 1; ezD ; Y ) as obtained from the commutative vertex algebra from (V ; ·; 1); D by `twisting' the vertex algebra operation Y by rz , making it noncommutative. 10 / 39 Dominic Joyce, Oxford University Lecture 5: More constructions of vertex algebras The coset and BRST constructions More constructions of vertex algebras The Borcherds bicharacter construction Monstrous Moonshine The orbifold construction Example 5.8 The Heisenberg vertex algebra V = C[x1; x2;:::] comes from the bicharacter construction by giving V the obvious algebra structure, coproduct generated by ∆(xn) = xn ⊗ 1 + 1 ⊗ xn, and derivation generated by D(xn) = nxn+1. One can show there is a unique ±1 −2 bicharacter rz : V × V ! C[z ] with rz (x1; x1) = z , and that the bicharacter construction yields the Heisenberg vertex algebra. I believe rz has the form, for cm;n 2 Q I didn't bother to compute 2 n X −m−n @ 0 o rz (p; q) = exp cm;nz p(x)q(x ) 0 0 xi = xj = 0, @xm@xn m;n>0 all i; j.
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