62,813 research outputs found
The Structure of a Graph Inverse Semigroup
Given any directed graph E one can construct a graph inverse semigroup G(E),
where, roughly speaking, elements correspond to paths in the graph. In this
paper we study the semigroup-theoretic structure of G(E). Specifically, we
describe the non-Rees congruences on G(E), show that the quotient of G(E) by
any Rees congruence is another graph inverse semigroup, and classify the G(E)
that have only Rees congruences. We also find the minimum possible degree of a
faithful representation by partial transformations of any countable G(E), and
we show that a homomorphism of directed graphs can be extended to a
homomorphism (that preserves zero) of the corresponding graph inverse
semigroups if and only if it is injective.Comment: 19 pages; corrected errors, improved organization, strengthened a
result (Theorem 20), added reference
Maximal subsemigroups of the semigroup of all mappings on an infinite set
In this paper we classify the maximal subsemigroups of the \emph{full
transformation semigroup} , which consists of all mappings on
the infinite set , containing certain subgroups of the symmetric group
\sym(\Omega) on . In 1965 Gavrilov showed that there are five maximal
subsemigroups of containing \sym(\Omega) when is
countable and in 2005 Pinsker extended Gavrilov's result to sets of arbitrary
cardinality.
We classify the maximal subsemigroups of on a set of
arbitrary infinite cardinality containing one of the following subgroups of
\sym(\Omega): the pointwise stabiliser of a non-empty finite subset of
, the stabiliser of an ultrafilter on , or the stabiliser of a
partition of into finitely many subsets of equal cardinality. If
is any of these subgroups, then we deduce a characterisation of the mappings
such that the semigroup generated by
equals .Comment: Revised according to comments by the referee, 29 pages, 11 figures,
to appear in Trans. American Mathematical Societ
Alternative Adaptive Filter Structures for Improved Radio Frequency Interference Cancellation in Radio Astronomy
In radio astronomy, reference signals from auxiliary antennas that receive
only the radio frequency interference (RFI) can be modified to model the RFI
environment at the astronomy receivers. The RFI can then be canceled from the
astronomy signal paths. However, astronomers typically only require signal
statistics. If the RFI statistics are changing slowly, the cancellation can be
applied to the signal correlations at a much lower rate than is required for
standard adaptive filters. In this paper we describe five canceler setups;
precorrelation and postcorrelation cancelers that use one or two reference
signals in different ways. The theoretical residual RFI and added noise levels
are examined and are demonstrated using microwave television RFI at the
Australia Telescope Compact Array. The RFI is attenuated to below the system
noise, a reduction of at least 20 dB. While dual-reference cancelers add more
reference noise than single-reference cancelers, this noise is zero-mean and
only adds to the system noise, decreasing the sensitivity. The residual RFI
that remains in the output of single-reference cancelers (but not
dual-reference cancelers) sets a nonzero noise floor that does not act like
random system noise and may limit the achievable sensitivity. Thus,
dual-reference cancelers often result in superior cancellation. Dual-reference
precorrelation cancelers require a double-canceler setup to be useful and to
give equivalent results to dual-reference postcorrelation cancelers.Comment: 11 pages created using emulateap
FORECAST OF INCOME AND WEALTH FOR THE FARM SECTOR, HOUSEHOLDS, AND THE FARMS THEY OPERATE
Consumer/Household Economics,
Advanced expander test bed program
The Advanced Expander Test Bed (AETB) is a key element in NASA's Space Chemical Engine Technology Program for development and demonstration of expander cycle oxygen/hydrogen engine and advanced component technologies applicable to space engines as well as launch vehicle upper stage engines. The AETB will be used to validate the high-pressure expander cycle concept, investigate system interactions, and conduct investigations of advanced mission focused components and new health monitoring techniques in an engine system environment. The split expander cycle AETB will operate at combustion chamber pressures up to 1200 psia with propellant flow rates equivalent to 20,000 lbf vacuum thrust. Contract work began 27 Apr. 1990. During 1992, a major milestone was achieved with the review of the final design of the oxidizer turbopump in Sep. 1992
Fish and freshwater crayfish in streams in the Cape Naturaliste region and Wilyabrup Brook
No abstract availabl
Theoretical investigation of the force and dynamically coupled torsional-axial-lateral dynamic response of eared rotors
Difficulties in solution methodology to be used to deal with the potentially higher nonlinear rotor equations when dynamic coupling is included. A solution methodology is selected to solve the nonlinear differential equations. The selected method was verified to give good results even at large nonlinearity levels. The transfer matrix methodology is extended to the solution of nonlinear problems
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