2,923 research outputs found
Carbon isotope fractionation during aerobic biodegradation of trichloroethene by Burkholderia cepacia G4: a tool to map degradation mechanisms
The strain Burkholderia cepacia G4 aerobically mineralized trichloroethene (TCE) to CO2 over a time period of similar to20 h. Three biodegradation experiments were conducted with different bacterial optical densities at 540 nm (OD(540)s) in order to test whether isotope fractionation was consistent. The resulting TCE degradation was 93, 83.8, and 57.2% (i.e., 7.0, 16.2, and 42.8% TCE remaining) at OD(540)s of 2.0, 1.1, and 0.6, respectively. ODs also correlated linearly with zero-order degradation rates (1.99, 1.11, and 0.64 mumol h(-1)). While initial nonequilibrium mass losses of TCE produced only minor carbon isotope shifts (expressed in per mille delta C- 13(VPDB)), they were 57.2, 39.6, and 17.0parts per thousand between the initial and final TCE levels for the three experiments, in decreasing order of their OD(540)s. Despite these strong isotope shifts, we found a largely uniform isotope fractionation. The latter is expressed with a Rayleigh enrichment factor, E, and was -18.2 when all experiments were grouped to a common point of 42.8% TCE remaining. Although, decreases of epsilon to -20.7 were observed near complete degradation, our enrichment factors were significantly more negative than those reported for anaerobic dehalogenation of TCE. This indicates typical isotope fractionation for specific enzymatic mechanisms that can help to differentiate between degradation pathways
A note on the differences of computably enumerable reals
We show that given any non-computable left-c.e. real Ī± there exists a left-c.e. real Ī² such that Ī±ā Ī²+Ī³ for all left-c.e. reals and all right-c.e. reals Ī³. The proof is non-uniform, the dichotomy being whether the given real Ī± is Martin-Loef random or not. It follows that given any universal machine U, there is another universal machine V such that the halting probability of U is not a translation of the halting probability of V by a left-c.e. real. We do not know if there is a uniform proof of this fact
Seven exercises planned to stimulate the flow of ideas in creative composition
Thesis (Ed.M.)--Boston Universit
A New Lower Bound on the Maximum Number of Satisfied Clauses in Max-SAT and its Algorithmic Applications
A pair of unit clauses is called conflicting if it is of the form ,
. A CNF formula is unit-conflict free (UCF) if it contains no pair
of conflicting unit clauses. Lieberherr and Specker (J. ACM 28, 1981) showed
that for each UCF CNF formula with clauses we can simultaneously satisfy at
least \pp m clauses, where \pp =(\sqrt{5}-1)/2. We improve the
Lieberherr-Specker bound by showing that for each UCF CNF formula with
clauses we can find, in polynomial time, a subformula with clauses
such that we can simultaneously satisfy at least \pp m+(1-\pp)m'+(2-3\pp)n"/2
clauses (in ), where is the number of variables in which are not in
.
We consider two parameterized versions of MAX-SAT, where the parameter is the
number of satisfied clauses above the bounds and . The
former bound is tight for general formulas, and the later is tight for UCF
formulas. Mahajan and Raman (J. Algorithms 31, 1999) showed that every instance
of the first parameterized problem can be transformed, in polynomial time, into
an equivalent one with at most variables and clauses. We improve
this to variables and clauses. Mahajan and Raman
conjectured that the second parameterized problem is fixed-parameter tractable
(FPT). We show that the problem is indeed FPT by describing a polynomial-time
algorithm that transforms any problem instance into an equivalent one with at
most variables. Our results are obtained using our improvement
of the Lieberherr-Specker bound above
Embedding structure matters: Comparing methods to adapt multilingual vocabularies to new languages
Pre-trained multilingual language models underpin a large portion of modern
NLP tools outside of English. A strong baseline for specializing these models
for specific languages is Language-Adaptive Pre-Training (LAPT). However,
retaining a large cross-lingual vocabulary and embedding matrix comes at
considerable excess computational cost during adaptation. In this study, we
propose several simple techniques to replace a cross-lingual vocabulary with a
compact, language-specific one. Namely, we address strategies for
re-initializing the token embedding matrix after vocabulary specialization. We
then provide a systematic experimental comparison of our techniques, in
addition to the recently-proposed Focus method. We demonstrate that: 1)
Embedding-replacement techniques in the monolingual transfer literature are
inadequate for adapting multilingual models. 2) Replacing cross-lingual
vocabularies with smaller specialized ones provides an efficient method to
improve performance in low-resource languages. 3) Simple embedding
re-initialization techniques based on script-wise sub-distributions rival
techniques such as Focus, which rely on similarity scores obtained from an
auxiliary model
Learning to translate by learning to communicate
We formulate and test a technique to use Emergent Communication (EC) with a
pretrained multilingual model to improve on modern Unsupervised NMT systems,
especially for low-resource languages. It has been argued that the currently
dominant paradigm in NLP of pretraining on text-only corpora will not yield
robust natural language understanding systems, and the need for grounded,
goal-oriented, and interactive language learning has been highlighted. In our
approach, we embed a modern multilingual model (mBART, Liu et. al. 2020) into
an EC image-reference game, in which the model is incentivized to use
multilingual generations to accomplish a vision-grounded task, with the
hypothesis that this will align multiple languages to a shared task space. We
present two variants of EC Fine-Tuning (Steinert-Threlkeld et. al. 2022), one
of which outperforms a backtranslation-based baseline in 6/8 translation
settings, and proves especially beneficial for the very low-resource languages
of Nepali and Sinhala
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Insect herbivory on seedlings of rainforest trees: effects of density and distance of conspecific and heterospecific neighbours
Natural enemies of plants such as insect herbivores can contribute to structuring and maintaining plant diversity in tropical forests. Most research in this area has focused on the role of specialized enemies and the extent to which herbivory on individual plant species is densityādependent. Relatively few insect herbivores specialize on a single host plant species. Insect herbivores that feed on more than one plant species may link the regeneration dynamics of their host species through āapparent competitionā or āapparent mutualism.ā We investigated herbivory and survival of seedlings of two tropical tree species (Cordia alliodora and Cordia bicolor) in the forests of Barro Colorado Island (Panama). We used experiments and observations to assess seedling fate in relation to the presence of conspecifics and heterospecifics across a range of spatial scales. Herbivory significantly increased seedling mortality and was highest at high local densities of C. alliodora seedlings. There was also evidence that high local densities of C. alliodora increased herbivory on coāoccurring C. bicolor seedlings. Synthesis. The elevated rates of seedling herbivory at high densities of conspecifics documented in our study are consistent with the predictions of the JanzenāConnell hypothesis, which explains how so many plant species can coexist in tropical forests. Our data also highlight the possibility that herbivoreāmediated densityādependence, facilitated by herbivores that feed on multiple plant species, can also occur across plant species. Enemyāmediated indirect effects of this sort have the potential to structure plant communities
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