133,407 research outputs found
Radicals, Metals and Magnetism
The interaction between unpaired electrons governs many physical properties of materials. Although in a fundamental sense the interaction is simple, a full understanding of the interaction in molecular systems is complicated by the presence of other bonding and non-bonding electrons. The resulting many body problem is very challenging. Nevertheless, much qualitative understanding can be obtained from applying simple molecular orbital theory and considering only the partly filled orbitals. The resulting model can be used to describe existing diradical and metal-radical systems and also has predicative value in the search for molecular magnets and design of nanoscale devices
Active Internet Traffic Filtering: Real-time Response to Denial of Service Attacks
Denial of Service (DoS) attacks are one of the most challenging threats to
Internet security. An attacker typically compromises a large number of
vulnerable hosts and uses them to flood the victim's site with malicious
traffic, clogging its tail circuit and interfering with normal traffic. At
present, the network operator of a site under attack has no other resolution
but to respond manually by inserting filters in the appropriate edge routers to
drop attack traffic. However, as DoS attacks become increasingly sophisticated,
manual filter propagation becomes unacceptably slow or even infeasible.
In this paper, we present Active Internet Traffic Filtering, a new automatic
filter propagation protocol. We argue that this system provides a guaranteed,
significant level of protection against DoS attacks in exchange for a
reasonable, bounded amount of router resources. We also argue that the proposed
system cannot be abused by a malicious node to interfere with normal Internet
operation. Finally, we argue that it retains its efficiency in the face of
continued Internet growth.Comment: Briefly describes the core ideas of AITF, a protocol for facing
Denial of Service Attacks. 6 pages lon
Average degree conditions forcing a minor
Mader first proved that high average degree forces a given graph as a minor.
Often motivated by Hadwiger's Conjecture, much research has focused on the
average degree required to force a complete graph as a minor. Subsequently,
various authors have consider the average degree required to force an arbitrary
graph as a minor. Here, we strengthen (under certain conditions) a recent
result by Reed and Wood, giving better bounds on the average degree required to
force an -minor when is a sparse graph with many high degree vertices.
This solves an open problem of Reed and Wood, and also generalises (to within a
constant factor) known results when is an unbalanced complete bipartite
graph
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