  A classical open problem in combinatorial geometry is to obtain tight
asymptotic bounds on the maximum number of k-level vertices in an arrangement
of n hyperplanes in d dimensions (vertices with exactly k of the hyperplanes
passing below them). This is a dual version of the k-set problem, which, in a
primal setting, seeks bounds for the maximum number of k-sets determined by n
points in d dimensions, where a k-set is a subset of size k that can be
separated from its complement by a hyperplane. The k-set problem is still wide
open even in the plane, with a substantial gap between the best known upper and
lower bounds. The gap gets larger as the dimension grows. In three dimensions,
the best known upper bound is O(nk^(3/2)).
  In its dual version, the problem can be generalized by replacing hyperplanes
by other families of surfaces (or curves in the planes). Reasonably sharp
bounds have been obtained for curves in the plane, but the known upper bounds
are rather weak for more general surfaces, already in three dimensions, except
for the case of triangles. The best known general bound, due to Chan is
O(n^2.997), for families of surfaces that satisfy certain (fairly weak)
properties.
  In this paper we consider the case of pseudoplanes in 3 dimensions (defined
in detail in the introduction), and establish the upper bound O(nk^(5/3)) for
the number of k-level vertices in an arrangement of n pseudoplanes. The bound
is obtained by establishing suitable (and nontrivial) extensions of dual
versions of classical tools that have been used in studying the primal k-set
problem, such as the Lova'sz Lemma and the Crossing Lemma.
