26,156 research outputs found
Delzant's T-invariant, Kolmogorov complexity and one-relator groups
We prove that ``almost generically'' for a one-relator group Delzant's
-invariant (which measures the smallest size of a finite presentation for a
group) is comparable in magnitude with the length of the defining relator. The
proof relies on our previous results regarding isomorphism rigidity of generic
one-relator groups and on the methods of the theory of Kolmogorov-Chaitin
complexity. We also give a precise asymptotic estimate (when is fixed and
goes to infinity) for the number of isomorphism classes of
-generator one-relator groups with a cyclically reduced defining relator of
length : Here
means that .Comment: A revised version, to appear in Comment. Math. Hel
Computational complexity of reconstruction and isomorphism testing for designs and line graphs
Graphs with high symmetry or regularity are the main source for
experimentally hard instances of the notoriously difficult graph isomorphism
problem. In this paper, we study the computational complexity of isomorphism
testing for line graphs of - designs. For this class of
highly regular graphs, we obtain a worst-case running time of for bounded parameters . In a first step, our approach
makes use of the Babai--Luks algorithm to compute canonical forms of
-designs. In a second step, we show that -designs can be reconstructed
from their line graphs in polynomial-time. The first is algebraic in nature,
the second purely combinatorial. For both, profound structural knowledge in
design theory is required. Our results extend earlier complexity results about
isomorphism testing of graphs generated from Steiner triple systems and block
designs.Comment: 12 pages; to appear in: "Journal of Combinatorial Theory, Series A
Construction of the discrete hull for the combinatorics of a regular pentagonal tiling of the plane
The article 'A "regular" pentagonal tiling of the plane' by P. L. Bowers and
K. Stephenson defines a conformal pentagonal tiling. This is a tiling of the
plane with remarkable combinatorial and geometric properties. However, it
doesn't have finite local complexity in any usual sense, and therefore we
cannot study it with the usual tiling theory. The appeal of the tiling is that
all the tiles are conformally regular pentagons. But conformal maps are not
allowable under finite local complexity. On the other hand, the tiling can be
described completely by its combinatorial data, which rather automatically has
finite local complexity. In this paper we give a construction of the discrete
hull just from the combinatorial data. The main result of this paper is that
the discrete hull is a Cantor space
Method for classification of the computational problems on the basis of the multifractal division of the complexity classes
This paper proposes the method of the multifractal division of the computational complexity classes, which is formalized by introducing the special equivalence relations on these classes. Exposing the self-similarity properties of the complexity classes structure, this method allows performing the accurate classification of the problems and demonstrates the capability of adaptation to the new advances in the computational complexity theory
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