  A new diagrammatic technique is developed to describe nonlocal effects (e.g.,
pseudogap formation) in the Hubbard-like models. In contrast to cluster
approaches, this method utilizes an exact transition to the dual set of
variables, and it therefore becomes possible to treat the irreducible vertices
of an effective {\it single-impurity} problem as small parameters. This
provides a very efficient interpolation between weak-coupling (band) and atomic
limits. The antiferromagnetic pseudogap formation in the Hubbard model is
correctly reproduced by just the lowest-order diagrams.
