Vertex operators appeared in the mathematics literature in 1978 , when
a representation of the affine Kac-Moody Lie algebra A[' — a central exten-
sion of the tensor product of s/(2, C) and the algebra of Laurent polynomials
in one variable — was constructed by means of differential operators now
understood as examples of Z2-twisted vertex operators. Previously, vertex
operators had been used by physicists in dual resonance models.
Within a few years, vertex operators formed a solid bridge between math-
ematics and physics. New constructions and results concerning finite groups,
Lie algebras and modular functions, and new models in string theory, quan-
tum field theory and statistical mechanics have contributed to the establish-
ment of the representation theory of vertex operator algebras — the algebraic
counterpart of two-dimensional conformal quantum field theory — as a cru-
cial area of research. The prototypical role of A[ ' and of its finite-dimensional
subalgebra 5/(2, C) in the study of more complicated symmetries is now un-
derstood in terms of vertex operator algebras and modules obtained from
root lattices of Lie algebras. Suitable vertex operator algebras carry natural
symmetries of two-dimensional conformal field theories and of the Monster
sporadic finite simple group.
In this spirit, this paper is a study of the main axiom for a vertex operator
algebra — the Jacobi identity (for vertex operators) — and its relation with
the standard Ax -modules. We extend the Jacobi identity for a vertex op-
erator algebra over a field of characteristic zero to multi-operator identities.
Next, based on constructions of C. Y. Dong and J. Lepowsky ( — ), we
establish a Jacobi identity for relative Z2-twisted vertex operators. Then in
the case of A[ , we naturally recover the twisted Z-operators and correspond-
ing generating function identities developed earlier by J. Lepowsky and R.
L. Wilson ( — ) for their construction of the standard At -modules.
This is accomplished by interpreting these identities as expansion coefficients
of multi-operator extensions of the relative Z2-twisted Jacobi identity.
The present work, which was stimulated by the book "Vertex Operator
Algebras and the Monster" by I. Frenkel, J. Lepowsky and A. Meurman 
and by a paper of R. E. Borcherds , is deeply rooted in Chapters 8 and
9 of  and in the work of C. Y. Dong and J. Lepowsky on relative vertex
operators, Z-algebras and parafermion algebras ( — ).