11,004 research outputs found
Sparse Kneser graphs are Hamiltonian
For integers and , the Kneser graph is the
graph whose vertices are the -element subsets of and whose
edges connect pairs of subsets that are disjoint. The Kneser graphs of the form
are also known as the odd graphs. We settle an old problem due to
Meredith, Lloyd, and Biggs from the 1970s, proving that for every ,
the odd graph has a Hamilton cycle. This and a known conditional
result due to Johnson imply that all Kneser graphs of the form
with and have a Hamilton cycle. We also prove that
has at least distinct Hamilton cycles for .
Our proofs are based on a reduction of the Hamiltonicity problem in the odd
graph to the problem of finding a spanning tree in a suitably defined
hypergraph on Dyck words
Preservation and decomposition theorems for bounded degree structures
We provide elementary algorithms for two preservation theorems for
first-order sentences (FO) on the class \^ad of all finite structures of degree
at most d: For each FO-sentence that is preserved under extensions
(homomorphisms) on \^ad, a \^ad-equivalent existential (existential-positive)
FO-sentence can be constructed in 5-fold (4-fold) exponential time. This is
complemented by lower bounds showing that a 3-fold exponential blow-up of the
computed existential (existential-positive) sentence is unavoidable. Both
algorithms can be extended (while maintaining the upper and lower bounds on
their time complexity) to input first-order sentences with modulo m counting
quantifiers (FO+MODm). Furthermore, we show that for an input FO-formula, a
\^ad-equivalent Feferman-Vaught decomposition can be computed in 3-fold
exponential time. We also provide a matching lower bound.Comment: 42 pages and 3 figures. This is the full version of: Frederik
Harwath, Lucas Heimberg, and Nicole Schweikardt. Preservation and
decomposition theorems for bounded degree structures. In Joint Meeting of the
23rd EACSL Annual Conference on Computer Science Logic (CSL) and the 29th
Annual ACM/IEEE Symposium on Logic in Computer Science (LICS), CSL-LICS'14,
pages 49:1-49:10. ACM, 201
Random-bit optimal uniform sampling for rooted planar trees with given sequence of degrees and Applications
In this paper, we redesign and simplify an algorithm due to Remy et al. for
the generation of rooted planar trees that satisfies a given partition of
degrees. This new version is now optimal in terms of random bit complexity, up
to a multiplicative constant. We then apply a natural process
"simulate-guess-and-proof" to analyze the height of a random Motzkin in
function of its frequency of unary nodes. When the number of unary nodes
dominates, we prove some unconventional height phenomenon (i.e. outside the
universal square root behaviour.)Comment: 19 page
Tight Limits on Nonlocality from Nontrivial Communication Complexity; a.k.a. Reliable Computation with Asymmetric Gate Noise
It has long been known that the existence of certain superquantum nonlocal
correlations would cause communication complexity to collapse. The absurdity of
a world in which any nonlocal binary function could be evaluated with a
constant amount of communication in turn provides a tantalizing way to
distinguish quantum mechanics from incorrect theories of physics; the statement
"communication complexity is nontrivial" has even been conjectured to be a
concise information-theoretic axiom for characterizing quantum mechanics. We
directly address the viability of that perspective with two results. First, we
exhibit a nonlocal game such that communication complexity collapses in any
physical theory whose maximal winning probability exceeds the quantum value.
Second, we consider the venerable CHSH game that initiated this line of
inquiry. In that case, the quantum value is about 0.85 but it is known that a
winning probability of approximately 0.91 would collapse communication
complexity. We show that the 0.91 result is the best possible using a large
class of proof strategies, suggesting that the communication complexity axiom
is insufficient for characterizing CHSH correlations. Both results build on new
insights about reliable classical computation. The first exploits our
formalization of an equivalence between amplification and reliable computation,
while the second follows from a rigorous determination of the threshold for
reliable computation with formulas of noise-free XOR gates and
-noisy AND gates.Comment: 64 pages, 6 figure
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