25,617 research outputs found
The RAM equivalent of P vs. RP
One of the fundamental open questions in computational complexity is whether
the class of problems solvable by use of stochasticity under the Random
Polynomial time (RP) model is larger than the class of those solvable in
deterministic polynomial time (P). However, this question is only open for
Turing Machines, not for Random Access Machines (RAMs).
Simon (1981) was able to show that for a sufficiently equipped Random Access
Machine, the ability to switch states nondeterministically does not entail any
computational advantage. However, in the same paper, Simon describes a
different (and arguably more natural) scenario for stochasticity under the RAM
model. According to Simon's proposal, instead of receiving a new random bit at
each execution step, the RAM program is able to execute the pseudofunction
, which returns a uniformly distributed random integer in the
range . Whether the ability to allot a random integer in this fashion is
more powerful than the ability to allot a random bit remained an open question
for the last 30 years.
In this paper, we close Simon's open problem, by fully characterising the
class of languages recognisable in polynomial time by each of the RAMs
regarding which the question was posed. We show that for some of these,
stochasticity entails no advantage, but, more interestingly, we show that for
others it does.Comment: 23 page
Gas depletion in Local Group dwarfs on ~250 kpc scales: Ram pressure stripping assisted by internal heating at early times
A recent survey of the Galaxy and M31 reveals that more than 90% of dwarf
galaxies within 270 kpc of their host galaxy are deficient in HI gas. At such
an extreme radius, the coronal halo gas is an order of magnitude too low to
remove HI gas through ram-pressure stripping for any reasonable orbit
distribution. However, all dwarfs are known to have an ancient stellar
population (\geq 10 Gyr) from early epochs of vigorous star formation which,
through heating of HI, could allow the hot halo to remove this gas. Our model
looks at the evolution of these dwarf galaxies analytically as the host-galaxy
dark matter halo and coronal halo gas builds up over cosmic time. The dwarf
galaxies - treated as spherically symmetric, smooth distributions of dark
matter and gas - experience early star formation, which sufficiently heats the
gas allowing it to be removed easily through tidal stripping by the host
galaxy, or ram-pressure stripping by a tenuous hot halo (n_H = 3x10^{-4}
cm^{-3} at 50 kpc). This model of evolution is able to explain the observed
radial distribution of gas-deficient and gas-rich dwarfs around the Galaxy and
M31 if the dwarfs fell in at high redshifts (z~3-10).Comment: ApJ accepted. 32 pages, 11 figure
Dynamical segregation of galaxies in groups and clusters
We have performed a systematic analysis of the dynamics of different galaxy
populations in galaxy groups from the 2dFGRS. For this purpose we have combined
all the groups into a single system, where velocities v and radius r are
expressed adimensionally. We have used several methods to compare the
distributions of relative velocities of galaxies with respect to the group
centre for samples selected according to their spectral type (as defined by
Madgwick et al., 2002), bj band luminosity and B-R colour index. We have found
strong segregation effects: spectral type I objects show a statistically
narrower velocity distribution than that of galaxies with a substantial star
formation activity (type II-IV). Similarly, the same behavior is observed for
galaxies with colour index B-R>1 compared to galaxies with B-R<1. Bright
(Mb-19) galaxies show the same segregation. It is not
important once the sample is restricted to a given spectral type. These effects
are particularly important in the central region (Rp<Rvirial/2) and do not have
a strong dependence on the mass of the parent group. These trends show a strong
correlation between the dynamics of galaxies in groups and star formation rate
reflected both by spectral type and by colour index.Comment: 7 pages, 8 figures. Accepted for publication in MNRA
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