4,172 research outputs found
A free subalgebra of the algebra of matroids
This paper is an initial inquiry into the structure of the Hopf algebra of
matroids with restriction-contraction coproduct. Using a family of matroids
introduced by Crapo in 1965, we show that the subalgebra generated by a single
point and a single loop in the dual of this Hopf algebra is free.Comment: 19 pages, 3 figures. Accepted for publication in the European Journal
of Combinatorics. This version incorporates a few minor corrections suggested
by the publisher
The Free product of Matroids
We introduce a noncommutative binary operation on matroids, called free
product. We show that this operation respects matroid duality, and has the
property that, given only the cardinalities, an ordered pair of matroids may be
recovered, up to isomorphism, from its free product. We use these results to
give a short proof of Welsh's 1969 conjecture, which provides a progressive
lower bound for the number of isomorphism classes of matroids on an n-element
set.Comment: 5 pages, 1 figure. Accepted for publication in the European Journal
of Combinatorics. See also arXiv:math.CO/040902
A unique factorization theorem for matroids
We study the combinatorial, algebraic and geometric properties of the free
product operation on matroids. After giving cryptomorphic definitions of free
product in terms of independent sets, bases, circuits, closure, flats and rank
function, we show that free product, which is a noncommutative operation, is
associative and respects matroid duality. The free product of matroids and
is maximal with respect to the weak order among matroids having as a
submatroid, with complementary contraction equal to . Any minor of the free
product of and is a free product of a repeated truncation of the
corresponding minor of with a repeated Higgs lift of the corresponding
minor of . We characterize, in terms of their cyclic flats, matroids that
are irreducible with respect to free product, and prove that the factorization
of a matroid into a free product of irreducibles is unique up to isomorphism.
We use these results to determine, for K a field of characteristic zero, the
structure of the minor coalgebra of a family of matroids that
is closed under formation of minors and free products: namely, is
cofree, cogenerated by the set of irreducible matroids belonging to .Comment: Dedicated to Denis Higgs. 25 pages, 3 figures. Submitted for
publication in the Journal of Combinatorial Theory (A). See
arXiv:math.CO/0409028 arXiv:math.CO/0409080 for preparatory work on this
subjec
51 Pegasi - a planet-bearing Maunder minimum candidate
We observed 51 Peg, the first detected planet-bearing star, in a 55 ks
XMM-Newton pointing and in 5 ks pointings each with Chandra HRC-I and ACIS-S.
The star has a very low count rate in the XMM observation, but is clearly
visible in the Chandra images due to the detectors' different sensitivity at
low X-ray energies. This allows a temperature estimate for 51 Peg's corona of
T<1MK; the detected ACIS-S photons can be plausibly explained by emission lines
of a very cool plasma near 200eV. The constantly low X-ray surface flux and the
flat-activity profile seen in optical CaII data suggest that 51 Peg is a
Maunder minimum star; an activity enhancement due to a Hot Jupiter, as proposed
by recent studies, seems to be absent. The star's X-ray fluxes in different
instruments are consistent with the exception of the HRC Imager, which might
have a larger effective area below 200eV than given in the calibration.Comment: accepted by A&
Quiescent and flaring X-ray emission from the nearby M/T dwarf binary SCR 1845-6357
We investigate an XMM-Newton observation of SCR 1845-6357, a nearby,
ultracool M8.5/T5.5 dwarf binary. The binary is unresolved in the XMM
detectors, however the X-ray emission is very likely from the M8.5 dwarf. We
compare its flaring emission to those of similar very low mass stars and
additionally present an XMM observation of the M8 dwarf VB 10. We detect
quasi-quiescent X-ray emission from SCR 1845-6357 at soft X-ray energies in the
0.2-2.0 keV band, as well as a strong flare with a count rate increase of a
factor of 30 and a duration of only 10 minutes. The quasi-quiescent X-ray
luminosity of log L_x = 26.2 erg/s and the corresponding activity level of log
L_x/L_bol = -3.8 point to a fairly active star. Coronal temperatures of up to 5
MK and frequent minor variability support this picture. During the flare, that
is accompanied by a significant brightening in the near-UV, plasma temperatures
of 25-30 MK are observed and an X-ray luminosity of L_x= 8 x 10^27 erg/s is
reached. SCR 1845-6357 is a nearby, very low mass star that emits X-rays at
detectable levels in quasi-quiescence, implying the existence of a corona. The
high activity level, coronal temperatures and the observed large flare point to
a rather active star, despite its estimated age of a few Gyr.Comment: Accepted by A&A, 6 pages, 5 figure
Coronal activity cycles in nearby G and K stars - XMM-Newton monitoring of 61 Cygni and Alpha Centauri
We use X-ray observations of the nearby binaries 61 Cyg A/B (K5V and K7V) and
Alpha Cen A/B (G2V and K1V) to study the long-term evolution of magnetic
activity in weakly to moderately active G + K dwarfs over nearly a decade.
Specifically we search for X-ray activity cycles and related coronal changes
and compare them to the solar behavior. For 61 Cyg A we find a regular coronal
activity cycle analog to its 7.3 yr chromospheric cycle. The X-ray brightness
variations are with a factor of three significantly lower than on the Sun, yet
the changes of coronal properties resemble the solar behavior with larger
variations occurring in the respective hotter plasma components. 61 Cyg B does
not show a clear cyclic coronal trend so far, but the X-ray data matches the
more irregular chromospheric cycle. Both Alpha Cen stars exhibit significant
long-term X-ray variability. Alpha Cen A shows indications for cyclic
variability of an order of magnitude with a period of about 12-15 years; the
Alpha Cen B data suggests an X-ray cycle with an amplitude of about six to
eight and a period of 8-9 years. The sample stars exhibit X-ray luminosities
ranging between Lx < 1x10^26 - 3x10^27 erg s^-1 in the 0.2-2.0 keV band and
have coronae dominated by cool plasma with variable average temperatures of
around 1.0-2.5 MK. We find that coronal activity cycles are apparently a common
phenomenon in older, slowly rotating G and K stars. The spectral changes of the
coronal X-ray emission over the cycles are solar-like in all studied targets.Comment: 11 pages, 9 figures, accepted by Astronomy and Astrophysic
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