17,959 research outputs found
On distinguishing trees by their chromatic symmetric functions
Let be an unrooted tree. The \emph{chromatic symmetric function} ,
introduced by Stanley, is a sum of monomial symmetric functions corresponding
to proper colorings of . The \emph{subtree polynomial} , first
considered under a different name by Chaudhary and Gordon, is the bivariate
generating function for subtrees of by their numbers of edges and leaves.
We prove that , where is the Hall inner
product on symmetric functions and is a certain symmetric function that
does not depend on . Thus the chromatic symmetric function is a stronger
isomorphism invariant than the subtree polynomial. As a corollary, the path and
degree sequences of a tree can be obtained from its chromatic symmetric
function. As another application, we exhibit two infinite families of trees
(\emph{spiders} and some \emph{caterpillars}), and one family of unicyclic
graphs (\emph{squids}) whose members are determined completely by their
chromatic symmetric functions.Comment: 16 pages, 3 figures. Added references [2], [13], and [15
Transferrin-polycation conjugates as carriers for DNA uptake into cells.
We have developed a high-efficiency nucleic acid delivery system that uses receptor-mediated endocytosis to carry DNA macromolecules into cells. We accomplished this by conjugating the iron-transport protein transferrin to polycations that bind nucleic acids. Human transferrin, as well as the chicken homologue conalbumin, has been covalently linked to the small DNA-binding protein protamine or to polylysines of various sizes through a disulfide linkage. These modified transferrin molecules maintain their ability to bind their cognate receptor and to mediate efficient iron transport into the cell. The transferrin-polycation molecules form electrophoretically stable complexes with double-stranded DNA, single-stranded DNA, and modified RNA molecules independent of nucleic acid size (from short oligonucleotides to DNA of 21 kilobase pairs). When complexes of transferrin-polycation and a bacterial plasmid DNA containing the gene for Photinus pyralis luciferase are supplied to eukaryotic cells, high-level expression of the luciferase gene occurs, demonstrating transferrin receptor-mediated endocytosis and expression of the imported DNA. We refer to this delivery system as "transferrinfection.
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Was Hurricane Sandy the âFat Tailâ of Climate Change?
Call them âblack swans,â âunknown unknowns,â âfat tails,â or â10-foot women.â Whatever you call them, theyâre bizarre events. They shouldnât happen. Usually they donât happen, but every once in a while they do happen, and then their impact makes daily events mere noise.
This week, one did happen. Sandy and its aftermath have all the characteristics that make fat tails such unique and frightening events.
Published in the Wall Street Journal Ideas Market on November 1, 2012
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Why Environmental Action Is so Hard
Gambles may be appealing in a casino, but in the real world humans
crave certainty.
Essay adapted from Climate Shock (Princeton University Press, 2015)
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What we know â and what we donât â about global warming
We take out car, fire and property insurances for much lower probabilities. Here we are talking about the whole planet, and we havenât shown willingness to insure ourselves. Meanwhile, we can, in fact, look at 11 degrees Fahrenheit and liken it to the planet âburningâ. Think of it as your body temperature: 98.6 degrees Fahrenheit is normal. Anything above 99.5 degrees Fahrenheit is a fever. Above 104 degrees Fahrenheit is life-threatening. Above 109.4 degrees Fahrenheit and you are dead or at least unconscious.
First published by PBS NewsHourâs Making Sen$e with Paul Solman. The accompanying NewsHour report aired on July 16, 2015
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How does climate stack up against other worst-case scenarios?
Climate isnât the only catastrophe threatening the planet, but it may be the one most in need of attention and resources.
The following is excerpted from Climate Shock (2015) by Gernot Wagner and Martin L. Weitzman. Published here with permission from Princeton University Press.
Published on Ensia.com on April 1st, 2015
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Playing God
The climate problem, twice over: Seven billion "free riders," meet the "free drivers" of geoengineering.
How serious is the threat of global warming? One way to figure out is to take your cues from some leading climate scientists: They have moved on. That doesnât mean theyâve abandoned the issue, but they are looking beyond what all agree is the most obvious solution â decreasing the amount of carbon we spew into the atmosphere in the first place. These scientists are beginning to look for a Plan B.
Published in Foreign Policy on October 24th, 2012
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Bailing out the planet
Mount Pinatubo, a volcano in the Philippines that had been dormant for over 400 years, began to rumble on April 2, 1991. Two months later, volcanic activity went into overdrive, culminating in a final explosion on June 15.
As a direct result of the volcanic eruption, global temperatures temporarily decreased by about 0.5°C (0.9°F), wiping out the entire temperature effects of human-caused global warming up to that point.
Mount Pinatubo did all that by spewing some 20 million tons of sulfur dioxide into the stratosphere. That amount counteracted the global warming effect of around 585 billion tons of carbon dioxide that humans had managed to put into the atmosphere by then.
Two years later, after most of the sulfur dioxide from Mount Pinatubo had washed out of the atmosphere, temperatures jumped back by the same 0.5°C and resumed growth where it left off.
Excerpted from âClimate Shock: The Economic Consequences of a Hotter Planetâ by Gernot Wagner and Martin L. Weitzman
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Climate Change: Like an Asteroid
The threat of climate change must be taken far more seriously, before it is too late.
If a civilization-as-we-know-it-altering asteroid were hurtling toward Earth, scheduled to hit a decade hence, and it had, say, a 5% chance of striking the planet, we would surely pull out all the stops to try to deflect its path.
This essay is adapted from âClimate Shock: The Economic Consequences of a Hotter Planetâ by Wagner and Weitzman (February 2015)
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The planet wonât notice you recycle, and your vote doesnât count
Put a group of economists in a room to debate the wisdom and virtue of individual action, and soon theyâll be debating the value of casting oneâs vote: zeroâin some strict, narrow, âeconomicâ sense of the world.
We canât leave things there. It would be both rather depressing and also rather narrow-minded. Maybe statistics and economics alone arenât the right tools with which to analyze oneâs personal action. Ethics, for one, plays an important role.
Excerpted from âClimate Shock: The Economic Consequences of a Hotter Planetâ by Gernot Wagner and Martin L. Weitzman
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