منابع مشابه
Making Markov chains less lazy
The mixing time of an ergodic, reversible Markov chain can be bounded in terms of the eigenvalues of the chain: specifically, the second-largest eigenvalue and the smallest eigenvalue. It has become standard to focus only on the secondlargest eigenvalue, by making the Markov chain “lazy”. (A lazy chain does nothing at each step with probability at least 1 2 , and has only nonnegative eigenvalue...
متن کاملFire forbids fifty-fifty forest
Recent studies have interpreted patterns of remotely sensed tree cover as evidence that forest with intermediate tree cover might be unstable in the tropics, as it will tip into either a closed forest or a more open savanna state. Here we show that across all continents the frequency of wildfires rises sharply as tree cover falls below ~40%. Using a simple empirical model, we hypothesize that t...
متن کاملAnchor Side Chains of Short Peptide Fragments Trigger Ligand-Exchange of Class II MHC Molecules
Class II MHC molecules display peptides on the cell surface for the surveillance by CD4+ T cells. To ensure that these ligands accurately reflect the content of the intracellular MHC loading compartment, a complex processing pathway has evolved that delivers only stable peptide/MHC complexes to the surface. As additional safeguard, MHC molecules quickly acquire a 'non-receptive' state once they...
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Heme oxygenases (HOs) are rate-limiting enzymes that catalyze the conversion of heme into equimolar amounts of biliverdin, carbon monoxide (CO), and iron.1 Biliverdin is subsequently reduced to bilirubin pigment by biliverdin reductase.2 The heme oxygenases consist of 2 major isoforms, HO-1 and HO-2. Whereas HO-2 is constitutively produced within the brain, testes, and the endothelium, HO-1 gen...
متن کاملTraumatic Optic Neuropathy [TON] - A Review
It is generally presumed that penetrating injuries of the orbit are associated with direct optic nerve injury. The incidence of TON after craniofacial trauma has been reported to be 0.5-1.5 % in older series. However, recent surveys report a higher figure; 2-5 % . This increased incidence may represent the diagnosis of subtle forms of TON closer to the time of injury. These types of cases appea...
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ژورنال
عنوان ژورنال: Scientific American
سال: 1902
ISSN: 0036-8733
DOI: 10.1038/scientificamerican06211902-429a