470 research outputs found
On the black-box complexity of Sperner's Lemma
We present several results on the complexity of various forms of Sperner's
Lemma in the black-box model of computing. We give a deterministic algorithm
for Sperner problems over pseudo-manifolds of arbitrary dimension. The query
complexity of our algorithm is linear in the separation number of the skeleton
graph of the manifold and the size of its boundary. As a corollary we get an
deterministic query algorithm for the black-box version of the
problem {\bf 2D-SPERNER}, a well studied member of Papadimitriou's complexity
class PPAD. This upper bound matches the deterministic lower
bound of Crescenzi and Silvestri. The tightness of this bound was not known
before. In another result we prove for the same problem an
lower bound for its probabilistic, and an
lower bound for its quantum query complexity, showing
that all these measures are polynomially related.Comment: 16 pages with 1 figur
Envy-free cake division without assuming the players prefer nonempty pieces
Consider players having preferences over the connected pieces of a cake,
identified with the interval . A classical theorem, found independently
by Stromquist and by Woodall in 1980, ensures that, under mild conditions, it
is possible to divide the cake into connected pieces and assign these
pieces to the players in an envy-free manner, i.e, such that no player strictly
prefers a piece that has not been assigned to her. One of these conditions,
considered as crucial, is that no player is happy with an empty piece. We prove
that, even if this condition is not satisfied, it is still possible to get such
a division when is a prime number or is equal to . When is at most
, this has been previously proved by Erel Segal-Halevi, who conjectured that
the result holds for any . The main step in our proof is a new combinatorial
lemma in topology, close to a conjecture by Segal-Halevi and which is
reminiscent of the celebrated Sperner lemma: instead of restricting the labels
that can appear on each face of the simplex, the lemma considers labelings that
enjoy a certain symmetry on the boundary
A Geometric Approach to Combinatorial Fixed-Point Theorems
We develop a geometric framework that unifies several different combinatorial
fixed-point theorems related to Tucker's lemma and Sperner's lemma, showing
them to be different geometric manifestations of the same topological
phenomena. In doing so, we obtain (1) new Tucker-like and Sperner-like
fixed-point theorems involving an exponential-sized label set; (2) a
generalization of Fan's parity proof of Tucker's Lemma to a much broader class
of label sets; and (3) direct proofs of several Sperner-like lemmas from
Tucker's lemma via explicit geometric embeddings, without the need for
topological fixed-point theorems. Our work naturally suggests several
interesting open questions for future research.Comment: 10 pages; an extended abstract appeared at Eurocomb 201
Decomposing 1-Sperner hypergraphs
A hypergraph is Sperner if no hyperedge contains another one. A Sperner
hypergraph is equilizable (resp., threshold) if the characteristic vectors of
its hyperedges are the (minimal) binary solutions to a linear equation (resp.,
inequality) with positive coefficients. These combinatorial notions have many
applications and are motivated by the theory of Boolean functions and integer
programming. We introduce in this paper the class of -Sperner hypergraphs,
defined by the property that for every two hyperedges the smallest of their two
set differences is of size one. We characterize this class of Sperner
hypergraphs by a decomposition theorem and derive several consequences from it.
In particular, we obtain bounds on the size of -Sperner hypergraphs and
their transversal hypergraphs, show that the characteristic vectors of the
hyperedges are linearly independent over the reals, and prove that -Sperner
hypergraphs are both threshold and equilizable. The study of -Sperner
hypergraphs is motivated also by their applications in graph theory, which we
present in a companion paper
Construction of Polar Codes with Sublinear Complexity
Consider the problem of constructing a polar code of block length for the
transmission over a given channel . Typically this requires to compute the
reliability of all the synthetic channels and then to include those that
are sufficiently reliable. However, we know from [1], [2] that there is a
partial order among the synthetic channels. Hence, it is natural to ask whether
we can exploit it to reduce the computational burden of the construction
problem.
We show that, if we take advantage of the partial order [1], [2], we can
construct a polar code by computing the reliability of roughly a fraction
of the synthetic channels. In particular, we prove that
is a lower bound on the number of synthetic channels to be
considered and such a bound is tight up to a multiplicative factor . This set of roughly synthetic channels is universal, in
the sense that it allows one to construct polar codes for any , and it can
be identified by solving a maximum matching problem on a bipartite graph.
Our proof technique consists of reducing the construction problem to the
problem of computing the maximum cardinality of an antichain for a suitable
partially ordered set. As such, this method is general and it can be used to
further improve the complexity of the construction problem in case a new
partial order on the synthetic channels of polar codes is discovered.Comment: 9 pages, 3 figures, presented at ISIT'17 and submitted to IEEE Trans.
Inform. Theor
- …