Twisted modules for vertex operator algebras
  This contribution is mainly based on joint papers with Lepowsky and Milas, and some parts of these papers are reproduced here. These papers further extended works by Lepowsky and by Milas. Following our joint papers, I explain the general principles of twisted modules for vertex operator algebras in their powerful formulation using formal series, and derive general relations satisfied by twisted and untwisted vertex operators. Using these, I prove new "equivalence" and "construction" theorems, identifying a set of sufficient conditions in order to have a twisted module for a vertex operator algebra, and a simple way of constructing the twisted vertex operator map. This essentially combines our general relations for twisted modules with ideas of Li (1996), who had obtained similar construction theorems using different relations. Then, I show how to apply these theorems in order to construct twisted modules for the Heisenberg vertex operator algebra. I obtain in a new way the explicit twisted vertex operator map, and in particular give a new derivation and expression for the formal operator $\Delta_x$ constructed some time ago by Frenkel, Lepowsky and Meurman. Finally, I reproduce parts of our joint papers. The untwisted relations in the Heisenberg vertex operator algebra are employed to explain properties of a certain central extension of a Lie algebra of differential operators on the circle, in relation to the Riemann Zeta function at negative integers. A family of representations for this algebra are constructed from twisted modules for the vertex operator algebra, and are related to the Bernoulli polynomials at rational values. 
Benjamin Doyon