The communication class
UPP
cc
is a communication analog of the Turing Machine complexity class
PP
. It is characterized by a matrix-analytic complexity measure called sign-rank (also called dimension complexity), and is essentially the most powerful communication class against which we know how to prove lower bounds.
For a communication problem
f
, let
f
∧
f
denote the function that evaluates
f
on two disjoint inputs and outputs the AND of the results. We exhibit a communication problem
f
with
UPP
cc
(
f
) =
O
(log
n
), and
UPP
cc
(
f
∧
f
) = Θ (log
2
n
). This is the first result showing that
UPP
communication complexity can increase by more than a constant factor under intersection. We view this as a first step toward showing that
UPP
cc
, the class of problems with polylogarithmic-cost
UPP
communication protocols, is not closed under intersection.
Our result shows that the function class consisting of intersections of two majorities on
n
bits has dimension complexity
n
Omega
Ω(log
n
)
. This matches an upper bound of (Klivans, O’Donnell, and Servedio, FOCS 2002), who used it to give a quasipolynomial time algorithm for PAC learning intersections of polylogarithmically many majorities. Hence, fundamentally new techniques will be needed to learn this class of functions in polynomial time.