5,212 research outputs found
Subgroups of direct products of limit groups
If are limit groups and is of
type \FP_n(\mathbb Q) then contains a subgroup of finite index that is
itself a direct product of at most limit groups. This settles a question of
Sela.Comment: 20 pages, no figures. Final version. Accepted by the Annals of
Mathematic
On the finite presentation of subdirect products and the nature of residually free groups
We establish {\em{virtual surjection to pairs}} (VSP) as a general criterion
for the finite presentability of subdirect products of groups: if
are finitely presented and
projects to a subgroup of finite index in
each , then is finitely presentable, indeed there
is an algorithm that will construct a finite presentation for .
We use the VSP criterion to characterise the finitely presented residually
free groups. We prove that the class of such groups is recursively enumerable.
We describe an algorithm that, given a finite presentation of a residually free
group, constructs a canonical embedding into a direct product of finitely many
limit groups. We solve the (multiple) conjugacy problem and membership problem
for finitely presentable subgroups of residually free groups. We also prove
that there is an algorithm that, given a finite generating set for such a
subgroup, will construct a finite presentation.
New families of subdirect products of free groups are constructed, including
the first examples of finitely presented subgroups that are neither
nor of Stallings-Bieri typeComment: 44 pages. To appear in American Journal of Mathematics. This is a
substantial rewrite of our previous Arxiv article 0809.3704, taking into
account subsequent developments, advice of colleagues and referee's comment
Finitely presented subgroups of automatic groups and their isoperimetric functions
We describe a general technique for embedding certain amalgamated products
into direct products. This technique provides us with a way of constructing a
host of finitely presented subgroups of automatic groups which are not even
asynchronously automatic. We can also arrange that such subgroups satisfy, at
best, an exponential isoperimetric inequality.Comment: DVI and Post-Script files only. To appear in J. London Math. So
Geology
Papers from private industry reporting applications of remote sensing to oil and gas exploration were presented. Digitally processed LANDSAT images were successfully employed in several geologic interpretations. A growing interest in digital image processing among the geologic user community was shown. The papers covered a wide geographic range and a wide technical and application range. Topics included: (1) oil and gas exploration, by use of radar and multisensor studies as well as by use of LANDSAT imagery or LANDSAT digital data, (2) mineral exploration, by mapping from LANDSAT and Skylab imagery and by LANDSAT digital processing, (3) geothermal energy studies with Skylab imagery, (4) environmental and engineering geology, by use of radar or LANDSAT and Skylab imagery, (5) regional mapping and interpretation, and digital and spectral methods
Ignition of thermally sensitive explosives between a contact surface and a shock
The dynamics of ignition between a contact surface and a shock wave is investigated using a
one-step reaction model with Arrhenius kinetics. Both large activation energy asymptotics and
high-resolution finite activation energy numerical simulations are employed. Emphasis is on comparing
and contrasting the solutions with those of the ignition process between a piston and a shock,
considered previously. The large activation energy asymptotic solutions are found to be qualitatively
different from the piston driven shock case, in that thermal runaway first occurs ahead of
the contact surface, and both forward and backward moving reaction waves emerge. These waves
take the form of quasi-steady weak detonations that may later transition into strong detonation
waves. For the finite activation energies considered in the numerical simulations, the results are
qualitatively different to the asymptotic predictions in that no backward weak detonation wave
forms, and there is only a weak dependence of the evolutionary events on the acoustic impedance
of the contact surface. The above conclusions are relevant to gas phase equation of state models.
However, when a large polytropic index more representative of condensed phase explosives is used,
the large activation energy asymptotic and finite activation energy numerical results are found to
be in quantitative agreement
Study of an advanced General Aviation Turbine Engine (GATE)
The best technology program for a small, economically viable gas turbine engine applicable to the general aviation helicopter and aircraft market for 1985-1990 was studied. Turboshaft and turboprop engines in the 112 to 746 kW (150 to 1000 hp) range and turbofan engines up to 6672 N (1500 lbf) thrust were considered. A good market for new turbine engines was predicted for 1988 providing aircraft are designed to capitalize on the advantages of the turbine engine. Parametric engine families were defined in terms of design and off-design performance, mass, and cost. These were evaluated in aircraft design missions selected to represent important market segments for fixed and rotary-wing applications. Payoff parameters influenced by engine cycle and configuration changes were aircraft gross mass, acquisition cost, total cost of ownership, and cash flow. Significant advantage over a current technology, small gas turbine engines was found especially in cost of ownership and fuel economy for airframes incorporating an air-cooled high-pressure ratio engine. A power class of 373 kW (500 hp) was recommended as the next frontier for technology advance where large improvements in fuel economy and engine mass appear possible through component research and development
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