Our main theoretical result is that, if a simple polytope has a pair of
complementary vertices (i.e., two vertices with no facets in common), then it
has a second such pair.
Normal surfaces are a key tool in computational knot theory and 3-manifold
topology, and have featured in significant computational breakthroughs in
recent years. Despite this, there has been little practical progress on
algorithms that use fundamental normal surfaces, which are described in terms
of a Hilbert basis for a pointed rational cone on a high-dimensional integer
lattice. In this paper we develop and implement several algorithms to enumerate
fundamental normal surfaces, by merging domain-specific techniques from normal
surface theory with classical Hilbert basis algorithms.
The enumeration of normal surfaces is a key bottleneck in computational
three-dimensional topology. The underlying procedure is the enumeration of
admissible vertices of a high-dimensional polytope, where admissibility is a
powerful but non-linear and non-convex constraint.
Normal surface theory is a central tool in algorithmic three-dimensional
topology, and the enumeration of vertex normal surfaces is the computational
bottleneck in many important algorithms. However, it is not well understood how
the number of such surfaces grows in relation to the size of the underlying
triangulation. Here we address this problem in both theory and practice. In
theory, we tighten the exponential upper bound substantially; furthermore, we
construct pathological triangulations that prove an exponential bound to be
unavoidable.
Given an arbitrary bitstream, we consider the problem of finding the longest
substring whose ratio of ones to zeroes equals a given value. The central
result of this paper is an algorithm that solves this problem in linear time.
The method involves (i) reformulating the problem as a constrained walk through
a sparse matrix, and then (ii) developing a data structure for this sparse
matrix that allows us to perform each step of the walk in amortised constant
time.
In this paper we develop a new test to help identify whether a closed,
orientable, irreducible 3-manifold is non-Haken. The test builds on work by
Jaco and Oertel, and also incorporates heuristic pruning techniques to test
whether a normal surface is compressible. As an application, we settle
Thurston's old question of whether the Weber-Seifert dodecahedral space is
non-Haken.
Normal and almost normal surfaces are essential tools for algorithmic
3-manifold topology, but to use them requires exponentially slow enumeration
algorithms in a high-dimensional vector space. The quadrilateral coordinates of
Tollefson alleviate this problem considerably for normal surfaces, by reducing
the dimension of this vector space from 7n to 3n (where n is the complexity of
the underlying triangulation). Here we develop an analogous theory for
octagonal almost normal surfaces, using quadrilateral and octagon coordinates
to reduce this dimension from 10n to 6n.