11,405 research outputs found
Strong Products of Hypergraphs: Unique Prime Factorization Theorems and Algorithms
It is well-known that all finite connected graphs have a unique prime factor
decomposition (PFD) with respect to the strong graph product which can be
computed in polynomial time. Essential for the PFD computation is the
construction of the so-called Cartesian skeleton of the graphs under
investigation.
In this contribution, we show that every connected thin hypergraph H has a
unique prime factorization with respect to the normal and strong (hypergraph)
product. Both products coincide with the usual strong graph product whenever H
is a graph. We introduce the notion of the Cartesian skeleton of hypergraphs as
a natural generalization of the Cartesian skeleton of graphs and prove that it
is uniquely defined for thin hypergraphs. Moreover, we show that the Cartesian
skeleton of hypergraphs can be determined in O(|E|^2) time and that the PFD can
be computed in O(|V|^2|E|) time, for hypergraphs H = (V,E) with bounded degree
and bounded rank
On the quaternion -isogeny path problem
Let \cO be a maximal order in a definite quaternion algebra over
of prime discriminant , and a small prime. We describe a
probabilistic algorithm, which for a given left -ideal, computes a
representative in its left ideal class of -power norm. In practice the
algorithm is efficient, and subject to heuristics on expected distributions of
primes, runs in expected polynomial time. This breaks the underlying problem
for a quaternion analog of the Charles-Goren-Lauter hash function, and has
security implications for the original CGL construction in terms of
supersingular elliptic curves.Comment: To appear in the LMS Journal of Computation and Mathematics, as a
special issue for ANTS (Algorithmic Number Theory Symposium) conferenc
Direct Product Primality Testing of Graphs is GI-hard
We investigate the computational complexity of the graph primality testing
problem with respect to the direct product (also known as Kronecker, cardinal
or tensor product). In [1] Imrich proves that both primality testing and a
unique prime factorization can be determined in polynomial time for (finite)
connected and nonbipartite graphs. The author states as an open problem how
results on the direct product of nonbipartite, connected graphs extend to
bipartite connected graphs and to disconnected ones. In this paper we partially
answer this question by proving that the graph isomorphism problem is
polynomial-time many-one reducible to the graph compositeness testing problem
(the complement of the graph primality testing problem). As a consequence of
this result, we prove that the graph isomorphism problem is polynomial-time
Turing reducible to the primality testing problem. Our results show that
connectedness plays a crucial role in determining the computational complexity
of the graph primality testing problem
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