54 research outputs found
Sufficient Conditions for Tuza's Conjecture on Packing and Covering Triangles
Given a simple graph , a subset of is called a triangle cover if
it intersects each triangle of . Let and denote the
maximum number of pairwise edge-disjoint triangles in and the minimum
cardinality of a triangle cover of , respectively. Tuza conjectured in 1981
that holds for every graph . In this paper, using a
hypergraph approach, we design polynomial-time combinatorial algorithms for
finding small triangle covers. These algorithms imply new sufficient conditions
for Tuza's conjecture on covering and packing triangles. More precisely,
suppose that the set of triangles covers all edges in . We
show that a triangle cover of with cardinality at most can be
found in polynomial time if one of the following conditions is satisfied: (i)
, (ii) , (iii)
.
Keywords: Triangle cover, Triangle packing, Linear 3-uniform hypergraphs,
Combinatorial algorithm
Bounds for graph regularity and removal lemmas
We show, for any positive integer k, that there exists a graph in which any
equitable partition of its vertices into k parts has at least ck^2/\log^* k
pairs of parts which are not \epsilon-regular, where c,\epsilon>0 are absolute
constants. This bound is tight up to the constant c and addresses a question of
Gowers on the number of irregular pairs in Szemer\'edi's regularity lemma.
In order to gain some control over irregular pairs, another regularity lemma,
known as the strong regularity lemma, was developed by Alon, Fischer,
Krivelevich, and Szegedy. For this lemma, we prove a lower bound of
wowzer-type, which is one level higher in the Ackermann hierarchy than the
tower function, on the number of parts in the strong regularity lemma,
essentially matching the upper bound. On the other hand, for the induced graph
removal lemma, the standard application of the strong regularity lemma, we find
a different proof which yields a tower-type bound.
We also discuss bounds on several related regularity lemmas, including the
weak regularity lemma of Frieze and Kannan and the recently established regular
approximation theorem. In particular, we show that a weak partition with
approximation parameter \epsilon may require as many as
2^{\Omega(\epsilon^{-2})} parts. This is tight up to the implied constant and
solves a problem studied by Lov\'asz and Szegedy.Comment: 62 page
Control over epitaxy and the role of the InAs/Al interface in hybrid two-dimensional electron gas systems
In-situ synthesised semiconductor/superconductor hybrid structures became an
important material platform in condensed matter physics. Their development
enabled a plethora of novel quantum transport experiments with focus on Andreev
and Majorana physics. The combination of InAs and Al has become the workhorse
material and has been successfully implemented in the form of one-dimensional
structures and two-dimensional electron gases. In contrast to the
well-developed semiconductor parts of the hybrid materials, the direct effect
of the crystal nanotexture of Al films on the electron transport still remains
unclear. This is mainly due to the complex epitaxial relation between Al and
the semiconductor. We present a study of Al films on shallow InAs
two-dimensional electron gas systems grown by molecular beam epitaxy, with
focus on control of the Al crystal structure. We identify the dominant grain
types present in our Al films and show that the formation of grain boundaries
can be significantly reduced by controlled roughening of the epitaxial
interface. Finally, we demonstrate that the implemented roughening does not
negatively impact either the electron mobility of the two-dimensional electron
gas or the basic superconducting properties of the proximitized system.Comment: 12 pages, 7 figures and supplementary materia
A Fractional Model of the Border Gateway Protocol (BGP)
The Border Gateway Protocol (BGP) is the interdomain routing protocol used to exchange routing information between Autonomous Systems (ASes) in the internet today. While intradomain routing protocols such as RIP are basically distributed algorithms for solving shortest path problems, the graph theoretic problem that BGP is trying to solve is called the stable paths problem (SPP). Unfortunately, unlike shortest path problems, it has been shown that instances of SPP can fail to have a solution and so BGP can fail to converge. We define a fractional version of SPP and show that all such instances of fractional SPP have solutions. We also show that while these solutions exist they are not necessarily half-integral
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