2,605 research outputs found
On the complexity of the Whitehead minimization problem
The Whitehead minimization problem consists in finding a minimum size element
in the automorphic orbit of a word, a cyclic word or a finitely generated
subgroup in a finite rank free group. We give the first fully polynomial
algorithm to solve this problem, that is, an algorithm that is polynomial both
in the length of the input word and in the rank of the free group. Earlier
algorithms had an exponential dependency in the rank of the free group. It
follows that the primitivity problem -- to decide whether a word is an element
of some basis of the free group -- and the free factor problem can also be
solved in polynomial time.Comment: v.2: Corrected minor typos and mistakes, improved the proof of the
main technical lemma (Statement 2.4); added a section of open problems. 30
page
Orbit decidability, applications and variations
We present the notion of orbit decidability into a more general framework,
exploring interesting generalizations and variations of this algorithmic
problem. A recent theorem by Bogopolski-Martino-Ventura gave a renovated
protagonism to this notion and motivated several interesting algebraic
applications
Orbit decidability and the conjugacy problem for some extensions of groups
Given a short exact sequence of groups with certain conditions, , we prove that has solvable conjugacy problem if and only if
the corresponding action subgroup is orbit decidable. From
this, we deduce that the conjugacy problem is solvable, among others, for all
groups of the form , , , and with virtually solvable action
group . Also, we give an easy way of constructing
groups of the form and with
unsolvable conjugacy problem. On the way, we solve the twisted conjugacy
problem for virtually surface and virtually polycyclic groups, and give an
example of a group with solvable conjugacy problem but unsolvable twisted
conjugacy problem. As an application, an alternative solution to the conjugacy
problem in is given
Algorithmic problems for free-abelian times free groups
We study direct products of free-abelian and free groups with special
emphasis on algorithmic problems. After giving natural extensions of standard
notions into that family, we find an explicit expression for an arbitrary
endomorphism of \ZZ^m \times F_n. These tools are used to solve several
algorithmic and decision problems for \ZZ^m \times F_n : the membership
problem, the isomorphism problem, the finite index problem, the subgroup and
coset intersection problems, the fixed point problem, and the Whitehead
problem.Comment: 38 page
Representation theory for high-rate multiple-antenna code design
Multiple antennas can greatly increase the data rate and reliability of a wireless communication link in a fading environment, but the practical success of using multiple antennas depends crucially on our ability to design high-rate space-time constellations with low encoding and decoding complexity. It has been shown that full transmitter diversity, where the constellation is a set of unitary matrices whose differences have nonzero determinant, is a desirable property for good performance. We use the powerful theory of fixed-point-free groups and their representations to design high-rate constellations with full diversity. Furthermore, we thereby classify all full-diversity constellations that form a group, for all rates and numbers of transmitter antennas. The group structure makes the constellations especially suitable for differential modulation and low-complexity decoding algorithms. The classification also reveals that the number of different group structures with full diversity is very limited when the number of transmitter antennas is large and odd. We, therefore, also consider extensions of the constellation designs to nongroups. We conclude by showing that many of our designed constellations perform excellently on both simulated and real wireless channels
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