204 research outputs found
Generating Bijections between HOAS and the Natural Numbers
A provably correct bijection between higher-order abstract syntax (HOAS) and
the natural numbers enables one to define a "not equals" relationship between
terms and also to have an adequate encoding of sets of terms, and maps from one
term family to another. Sets and maps are useful in many situations and are
preferably provided in a library of some sort. I have released a map and set
library for use with Twelf which can be used with any type for which a
bijection to the natural numbers exists.
Since creating such bijections is tedious and error-prone, I have created a
"bijection generator" that generates such bijections automatically together
with proofs of correctness, all in the context of Twelf.Comment: In Proceedings LFMTP 2010, arXiv:1009.218
Cassini UVIS Observations of the Io Plasma Torus. IV. Modeling Temporal and Azimuthal Variability
In this fourth paper in a series, we present a model of the remarkable
temporal and azimuthal variability of the Io plasma torus observed during the
Cassini encounter with Jupiter. Over a period of three months, the Cassini
Ultraviolet Imaging Spectrograph (UVIS) observed a dramatic variation in the
average torus composition. Superimposed on this long-term variation, is a
10.07-hour periodicity caused by an azimuthal variation in plasma composition
subcorotating relative to System III longitude. Quite surprisingly, the
amplitude of the azimuthal variation appears to be modulated at the beat
frequency between the System III period and the observed 10.07-hour period.
Previously, we have successfully modeled the months-long compositional change
by supposing a factor of three increase in the amount of material supplied to
Io's extended neutral clouds. Here, we extend our torus chemistry model to
include an azimuthal dimension. We postulate the existence of two azimuthal
variations in the number of super-thermal electrons in the torus: a primary
variation that subcorotates with a period of 10.07 hours and a secondary
variation that remains fixed in System III longitude. Using these two hot
electron variations, our model can reproduce the observed temporal and
azimuthal variations observed by Cassini UVIS.Comment: Revised 24 August 2007 Accepted by Icarus, 50 pages, 2 Tables, 8
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Explicit Substitutions for Contextual Type Theory
In this paper, we present an explicit substitution calculus which
distinguishes between ordinary bound variables and meta-variables. Its typing
discipline is derived from contextual modal type theory. We first present a
dependently typed lambda calculus with explicit substitutions for ordinary
variables and explicit meta-substitutions for meta-variables. We then present a
weak head normalization procedure which performs both substitutions lazily and
in a single pass thereby combining substitution walks for the two different
classes of variables. Finally, we describe a bidirectional type checking
algorithm which uses weak head normalization and prove soundness.Comment: In Proceedings LFMTP 2010, arXiv:1009.218
Comparisons between MHD model calculations and observations of Cassini flybys of Titan
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/94689/1/jgra18227.pd
Ultraviolet emissions from the magnetic footprints of Io, Ganymede and Europa on Jupiter
Io leaves a magnetic footprint on Jupiter's upper atmosphere that appears as a spot of ultraviolet emission that remains fixed underneath Io as Jupiter rotates(1-3). The specific physical mechanisms responsible for generating those emissions are not well understood, but in general the spot seems to arise because of an electromagnetic interaction between Jupiter's magnetic field and the plasma surrounding Io, driving currents of around 1 million amperes down through Jupiter's ionosphere(4-6). The other galilean satellites may also leave footprints, and the presence or absence of such footprints should illuminate the underlying physical mechanism by revealing the strengths of the currents linking the satellites to Jupiter. Here we report persistent, faint, far-ultraviolet emission from the jovian footprints of Ganymede and Europa. We also show that Io's magnetic footprint extends well beyond the immediate vicinity of Io's flux-tube interaction with Jupiter, and much farther than predicted theoretically(4-6); the emission persists for several hours downstream. We infer from these data that Ganymede and Europa have persistent interactions with Jupiter's magnetic field despite their thin atmospheres.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/62861/1/415997a.pd
The Importance of Exploring Neptune’s Aurora and Ionosphere
Whitepaper #261 submitted to the Planetary Science and Astrobiology Decadal Survey 2023-2032. Topics: atmospheric/exospheric evolution; giant planets systems
Neptune’s aurorae are some of the least well understood within the entire solar system. Observing them with in-situ space missions will greatly improve our understanding of the aurorae and their atmospheric and magnetospheric context, and will provide a new understanding of all Ice Giant aurorae, both within the solar system and around other stars
Importance of RNA-protein interactions in bacterial ribonuclease P structure and catalysis
Ribonuclease P (RNase P) is a ribonucleoprotein (RNP) complex that catalyzes the metal-dependent maturation of the 5′ end of precursor tRNAs (pre-tRNAs) in all organisms. RNase P is comprised of a catalytic RNA (P RNA), and at least one essential protein (P protein). Although P RNA is the catalytic subunit of the enzyme and is active in the absence of P protein under high salt concentrations in vitro, the protein is still required for enzyme activity in vivo. Therefore, the function of the P protein and how it interacts with both P RNA and pre-tRNA have been the focus of much ongoing research. RNA-protein interactions in RNase P serve a number of critical roles in the RNP including stabilizing the structure, and enhancing the affinity for substrates and metal ions. This review examines the role of RNA-protein interactions in bacterial RNase P from both structural and mechanistic perspectives. © 2007 Wiley Periodicals, Inc. Biopolymers 87: 329–338, 2007. This article was originally published online as an accepted preprint. The “Published Online” date corresponds to the preprint version. You can request a copy of the preprint by emailing the Biopolymers editorial office at [email protected] Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/57327/1/20846_ftp.pd
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