Using general principles in the theory of vertex operator algebras and their
twisted modules, we obtain a bosonic, twisted construction of a certain central
extension of a Lie algebra of differential operators on the circle, for an
arbitrary twisting automorphism. The construction involves the Bernoulli
polynomials in a fundamental way. We develop new identities and principles in
the theory of vertex operator algebras and their twisted modules, and explain
the construction by applying general results, including an identity that we
call "modified weak associativity", to the Heisenberg vertex operator algebra.
This paper gives proofs and further explanations of results announced earlier.
It is a generalization to twisted vertex operators of work announced by the
second author some time ago, and includes as a special case the proof of the
main results of that work.