26,675 research outputs found
Computational Complexity for Physicists
These lecture notes are an informal introduction to the theory of
computational complexity and its links to quantum computing and statistical
mechanics.Comment: references updated, reprint available from
http://itp.nat.uni-magdeburg.de/~mertens/papers/complexity.shtm
An Algorithm to Compute the Topological Euler Characteristic, Chern-Schwartz-MacPherson Class and Segre Class of Projective Varieties
Let be a closed subscheme of a projective space . We give
an algorithm to compute the Chern-Schwartz-MacPherson class, Euler
characteristic and Segre class of . The algorithm can be implemented using
either symbolic or numerical methods. The algorithm is based on a new method
for calculating the projective degrees of a rational map defined by a
homogeneous ideal. Using this result and known formulas for the
Chern-Schwartz-MacPherson class of a projective hypersurface and the Segre
class of a projective variety in terms of the projective degrees of certain
rational maps we give algorithms to compute the Chern-Schwartz-MacPherson class
and Segre class of a projective variety. Since the Euler characteristic of
is the degree of the zero dimensional component of the
Chern-Schwartz-MacPherson class of our algorithm also computes the Euler
characteristic . Relationships between the algorithm developed here
and other existing algorithms are discussed. The algorithm is tested on several
examples and performs favourably compared to current algorithms for computing
Chern-Schwartz-MacPherson classes, Segre classes and Euler characteristics
On dynamic breadth-first search in external-memory
We provide the first non-trivial result on dynamic breadth-first search (BFS) in external-memory: For general sparse undirected graphs of initially nodes and O(n) edges and monotone update sequences of either edge insertions or edge deletions, we prove an amortized high-probability bound of O(n/B^{2/3}+\sort(n)\cdot \log B) I/Os per update. In contrast, the currently best approach for static BFS on sparse undirected graphs requires \Omega(n/B^{1/2}+\sort(n)) I/Os. 1998 ACM Subject Classification: F.2.2. Key words and phrases: External Memory, Dynamic Graph Algorithms, BFS, Randomization
Connectivity Oracles for Graphs Subject to Vertex Failures
We introduce new data structures for answering connectivity queries in graphs
subject to batched vertex failures. A deterministic structure processes a batch
of failed vertices in time and thereafter
answers connectivity queries in time. It occupies space . We develop a randomized Monte Carlo version of our data structure
with update time , query time , and space
for any failure bound . This is the first connectivity oracle for
general graphs that can efficiently deal with an unbounded number of vertex
failures.
We also develop a more efficient Monte Carlo edge-failure connectivity
oracle. Using space , edge failures are processed in time and thereafter, connectivity queries are answered in
time, which are correct w.h.p.
Our data structures are based on a new decomposition theorem for an
undirected graph , which is of independent interest. It states that
for any terminal set we can remove a set of
vertices such that the remaining graph contains a Steiner forest for with
maximum degree
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