23,059 research outputs found
Representability of Hom implies flatness
Let be a projective scheme over a noetherian base scheme , and let
be a coherent sheaf on . For any coherent sheaf on , consider the
set-valued contravariant functor on -schemes, defined by
where and are the pull-backs of
and to . A basic result of Grothendieck ([EGA] III
7.7.8, 7.7.9) says that if is flat over then is
representable for all . We prove the converse of the above, in fact, we show
that if is a relatively ample line bundle on over such that the
functor is representable for infinitely many positive integers
, then is flat over . As a corollary, taking , it follows that
if is a coherent sheaf on then the functor on
the category of -schemes is representable if and only if is locally free
on . This answers a question posed by Angelo Vistoli. The techniques we use
involve the proof of flattening stratification, together with the methods used
in proving the author's earlier result (see arXiv.org/abs/math.AG/0204047) that
the automorphism group functor of a coherent sheaf on is representable if
and only if the sheaf is locally free.Comment: 9 pages, LaTe
Planetary Nebula Abundances and Morphology: Probing the Chemical Evolution of the Milky Way
This paper presents a homogeneous study of abundances in a sample of 79
northern galactic planetary nebulae whose morphological classes have been
uniformly determined. Ionic abundances and plasma diagnostics were derived from
selected optical line strengths in the literature, and elemental abundances
were estimated with the Ionization Correction Factor developed by Kingsbourgh &
Barlow (1994). We compare the elemental abundances to the final yields obtained
from stellar evolution models of low-and intermediate-mass stars, and we
confirm that most Bipolar planetary nebulae have high nitrogen and helium
abundance, and are the likely progeny of stars with main-sequence mass larger
than 3 solar masses. We derive =0.27, and discuss the implication of such
a high ratio in connection with the solar neon abundance. We determine the
galactic gradients of oxygen and neon, and found Delta log (O/H)/Delta R=-0.01
dex/kpc$ and Delta log (Ne/H)/Delta R=-0.01 dex/kpc. These flat PN gradients do
not reconcile with galactic metallicity gradients flattening with time.Comment: The Astrophysical Journal, in pres
Circumstellar Interaction Around Type Ib/c Supernovae and the GRB Connection
Radio observations of Type Ib/c supernovae suggest that circumstellar
interaction takes place with a wide range of wind densities, comparable to that
seen in Galactic Wolf-Rayet stars. Efficient production of magnetic field in
the shocked region is needed. The X-ray emission observed from some Type Ib/c
supernovae is higher than would be expected by the thermal or inverse Compton
mechanisms; a synchrotron interpretation requires a flattening of the electron
energy spectrum at high energies, as might occur in a cosmic ray dominated
shock wave. The wind density variations that are indicated in two supernovae
may be due to a binary companion, although variable mass loss from a single
star remains a possibility. Other than the optical supernova radiation, the
emission from the nearby SN 2006aj/GRB 060218 appears to be powered by a
central engine, while that from SN 1998bw/GRB 980425 could be powered by either
a central engine or the outer supernova ejecta.Comment: 8 pages, to appear in Circumstellar Media and Late Stages of Massive
Stellar Evolution, RevMexA
A Note on the Expressiveness of BIP
We extend our previous algebraic formalisation of the notion of
component-based framework in order to formally define two forms, strong and
weak, of the notion of full expressiveness. Our earlier result shows that the
BIP (Behaviour-Interaction-Priority) framework does not possess the strong full
expressiveness. In this paper, we show that BIP has the weak form of this
notion and provide results detailing weak and strong full expressiveness for
classical BIP and several modifications, obtained by relaxing the constraints
imposed on priority models.Comment: In Proceedings EXPRESS/SOS 2016, arXiv:1608.0269
ASSOCIATIONS BETWEEN EXERCISE-INDUCED FLATTENING OF FOOT ARCH AND FATIGUE OF EXTRINSIC AND INTRINSIC FOOT MUSCLES
We aimed to examine the associations between exercise-induced flattening of the foot arch and fatigue of extrinsic and intrinsic foot muscles. Fifteen male participants repeated 100 hops/set until they could no longer maintain the hop frequency or had completed 30 sets. The sagittal plane range of motion (ROM) of the midfoot during stance phase significantly decreased at the final set compared to the initial set (-8.8°). After the fatigue task, MRI T2 relaxation time in all measured extrinsic and intrinsic foot muscles significantly increased (17.2‒37.4%); however, only its increase in the tibialis posterior (TP) correlated with the relative change in midfoot ROM (r = 0.684). These results suggest that fatigue of TP is associated with the occurrence of flattening of the foot arch. This study provides a clinical implication that monitoring midfoot kinematics can be used to assess the condition of TP
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