15,363 research outputs found
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
A recursive presentation for Mihailova's subgroup
We give an explicit recursive presentation for Mihailova's subgroup of
corresponding to a finite, concise and Peiffer aspherical
presentation . This partially answers a
question of R.I. Grigorchuk, [8, Problem 4.14]. As a corollary, we construct a
finitely generated recursively presented orbit undecidable subgroup of
.Comment: 9 page
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
Concepts in cardiology : a historical perspective
Our current knowledge of the anatomy and physiology of the circulatory system has been
reached by deduction and reasoning over several centuries. In this article, the history of these theories is briefly outlined.peer-reviewe
Slow and fast components in the X-ray light curves of Gamma-Ray Bursts
Gamma-ray burst light curves show quite different patterns: from very simple
to extremely complex. We present a temporal and spectral study of the light
curves in three energy bands (2-5, 5-10, 10-26 keV) of ten GRBs detected by the
Wide Field Cameras on board BeppoSAX. For some events the time profiles are
characterized by peaks superposed on a slowly evolving pedestal, which in some
cases becomes less apparent at higher energies. We describe this behaviour with
the presence of two components (slow and fast) having different variability
time scales. We modelled the time evolution of slow components by means of an
analytical function able to describe asymmetric rising and decaying profiles.
The residual light curves, after the subtraction of the slow components,
generally show structures more similar to the original curves in the highest
energy band. Spectral study of these two components was performed evaluating
their hardness ratios, used also to derive photon indices. Slow components are
found generally softer than the fast ones suggesting that their origin is
likely different. Being typical photon indices lower than those of the
afterglows there is no evidence that the emission processes are similar.
Another interesting possibility is that slow components can be related to the
presence of a hot photosphere having a thermal spectrum with kT around a few
keV superposed to a rapid variable non-thermal emission of the fast component.Comment: 16 pages, 20 figures (18 color, 2 B&W), accepted for publication in
Astronomy and Astrophysic
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