Showing 1741 - 1750 of 2505 Items
Combinatorial properties of Thompson's group F
Date: 2004-07-01
Creator: Sean Cleary
Jennifer Taback
Access: Open access
- We study some combinatorial consequences of Blake Fordham's theorems on the word metric of Thompson's group F in the standard two generator presentation. We explore connections between the tree pair diagram representing an element w of F, its normal form in the infinite presentation, its word length, and minimal length representatives of it. We estimate word length in terms of the number and type of carets in the tree pair diagram and show sharpness of those estimates. In addition we explore some properties of the Cayley graph of F with respect to the two generator finite presentation. Namely, we exhibit the form of "dead end" elements in this Cayley graph, and show that it has no "deep pockets". Finally, we discuss a simple method for constructing minimal length representatives for strictly positive or negative words.
Functional and mutational analysis of the light-harvesting chlorophyll a/b protein of thylakoid membranes.
Date: 1986-01-01
Creator: B. D. Kohorn
E. Harel
P. R. Chitnis
J. P. Thornber
E. M., Tobin
Access: Open access
- The precursor for a Lemna light-harvesting chlorophyll a/b protein (pLHCP) has been synthesized in vitro from a single member of the nuclear LHCP multigene family. We report the sequence of this gene. When incubated with Lemna chloroplasts, the pLHCP is imported and processed into several polypeptides, and the mature form is assembled into the light-harvesting complex of photosystem II (LHC II). The accumulation of the processed LHCP is enhanced by the addition to the chloroplasts of a precursor and a co-factor for chlorophyll biosynthesis. Using a model for the arrangement of the mature polypeptide in the thylakoid membrane as a guide, we have created mutations that lie within the mature coding region. We have studied the processing, the integration into thylakoid membranes, and the assembly into light-harvesting complexes of six of these deletions. Four different mutant LHCPs are found as processed proteins in the thylakoid membrane, but only one appears to have an orientation in the membrane that is similar to that of the wild type. No mutant LHCP appears in LHC II. The other two mutant LHCPs cannot be detected within the chloroplasts. We conclude that stable complex formation is not required for the processing and insertion of altered LHCPs into the thylakoid membrane. We discuss the results in light of our model.
One-instanton test of a Seiberg-Witten curve from M-theory: The antisymmetric representation of SU(N)
Date: 1998-11-16
Creator: Stephen G. Naculich
Henric Rhedin
Howard J. Schnitzer
Access: Open access
- One-instanton predictions are obtained from the Seiberg-Witten curve derived from M-theory by Landsteiner and Lopez for the Coulomb branch of N = 2 supersymmetric SU(N) gauge theory with a matter hypermultiplet in the antisymmetric representation. Since this cubic curve describes a Riemann surface that is non-hyperelliptic, a systematic perturbation expansion about a hyperelliptic curve is developed, with a comparable expansion for the Seiberg-Witten differential. Calculation of the period integrals of the SW differential by the method of residues of D'Hoker, Krichever, and Phong enables us to compute the prepotential explicitly to one-instanton order. It is shown that the one-instanton predictions for SU(2), SU(3), and SU(4) agree with previously available results. For SU(N), N ≥ 5, our analysis provides explicit predictions of a curve derived from M-theory at the one-instanton level in field theory. © 1998 Elsevier Science B.V.
Evaluation of the free energy of two-dimensional Yang-Mills theory
Date: 1996-01-01
Creator: Michael Crescimanno
Stephen G. Naculich
Howard J. Schnitzer
Access: Open access
- The free energy in the weak-coupling phase of two-dimensional Yang-Mills theory on a sphere for [Formula presented] and [Formula presented] is evaluated in the [Formula presented] expansion using the techniques of Gross and Matytsin. Many features of Yang-Mills theory are universal among different gauge groups in the large [Formula presented] limit, but significant differences arise in subleading order in [Formula presented]. © 1996 The American Physical Society.
String calculation of QCD Wilson loops on arbitrary surfaces
Date: 1995-01-01
Creator: Stephen G. Naculich
Harold A. Riggs
Access: Open access
- Compact string expressions are found for nonintersecting Wilson loops in SU(N) Yang-Mills theory on any surface (orientable or nonorientable) as a weighted sum over covers of the surface. All terms from the coupled chiral sectors of the 1/N expansion of the Wilson loop expectation values are included. © 1995 The American Physical Society.
Social Learning Strategies: Bridge-Building between Fields
Date: 2018-07-01
Creator: Rachel L. Kendal
Neeltje J. Boogert
Luke Rendell
Kevin N. Laland
Mike, Webster
Patricia L. Jones
Access: Open access
- While social learning is widespread, indiscriminate copying of others is rarely beneficial. Theory suggests that individuals should be selective in what, when, and whom they copy, by following 'social learning strategies’ (SLSs). The SLS concept has stimulated extensive experimental work, integrated theory, and empirical findings, and created impetus to the social learning and cultural evolution fields. However, the SLS concept needs updating to accommodate recent findings that individuals switch between strategies flexibly, that multiple strategies are deployed simultaneously, and that there is no one-to-one correspondence between psychological heuristics deployed and resulting population-level patterns. The field would also benefit from the simultaneous study of mechanism and function. SLSs provide a useful vehicle for bridge-building between cognitive psychology, neuroscience, and evolutionary biology.
Consequences of toxic secondary compounds in nectar for mutualist bees and antagonist butterflies
Date: 2016-10-01
Creator: Patricia L. Jones
Anurag A. Agrawal
Access: Open access
- Attraction of mutualists and defense against antagonists are critical challenges for most organisms and can be especially acute for plants with pollinating and non-pollinating flower visitors. Secondary compounds in flowers have been hypothesized to adaptively mediate attraction of mutualists and defense against antagonists, but this hypothesis has rarely been tested. The tissues of milkweeds (Asclepias spp.) contain toxic cardenolides that have long been studied as chemical defenses against herbivores. Milkweed nectar also contains cardenolides, and we have examined the impact of manipulating cardenolides in nectar on the foraging choices of two flower visitors: generalist bumble bees, Bombus impatiens, which are mutualistic pollinators, and specialist monarch butterflies, Danaus plexippus, which are herbivores as larvae and ineffective pollinators as adults. Although individual bumble bees in single foraging bouts showed no avoidance of cardenolides at the highest natural concentrations reported for milkweeds, a pattern of deterrence did arise when entire colonies were allowed to forage for several days. Monarch butterflies were not deterred by the presence of cardenolides in nectar when foraging from flowers, but laid fewer eggs on plants paired with cardenolide-laced flowers compared to controls. Thus, although deterrence of bumble bees by cardenolides may only occur after extensive foraging, a primary effect of nectar cardenolides appears to be reduction of monarch butterfly oviposition.
Interview with Joan Pedersen by Andrea L’Hommedieu
Date: 2008-11-26
Creator: Joan S Pedersen
Access: Open access
- Biographical NoteJoan (Speed) Pedersen was born on February 11, 1940, in Boston, Massachusetts. Her mother was a legal secretary for an attorney’s office and her father worked in distribution for Firestone Tire. She grew up in West Roxbury, a heavily Irish Catholic part of Boston. She married and moved to Cape Cod, and later to Maine. From 1982-1984, she worked in Senator Mitchell’s field office in Lewiston, Maine, serving constituents. She later worked for Senator William S. Cohen and Representative John E. Baldacci. SummaryInterview includes discussion of: growing up in Boston in the 1940s and 1950s; work as a caseworker in Senator Mitchell’s field office; heart transplant case for a veteran; gun control and abortion issues dealt with in the office; the relationships among Mitchell’s staff; Senator Mitchell’s reputation; Pedersen’s interactions with the Senator Mitchell; anecdote about her teenage daughter answering her phone when the Mitchell called; interaction between the Washington, D.C. staff and the Maine staff; daughter’s work in Northern Ireland; increased workload for caseworkers when Mitchell became majority leader; arrival of computers in the office; roles of the press secretary and the scheduler; state staff’s serving as the Mitchell’s eyes and ears; office security; transition to working for Senator Cohen after Mitchell left the Senate; Mitchell’s and Cohen’s working relationship; Mitchell’s qualifications for playing a role in the new administration; the value of bipartisanship and how Maine politicians have exemplified it; Mitchell’s confidence in his staff; and Pedersen’s feelings about having worked for Mitchell.
Interview with Warren Rudman by Brien Williams
Date: 2010-06-02
Creator: Warren B Rudman
Access: Open access
- Biographical NoteWarren B. Rudman was born on May 18, 1930, and served as a Republican in the U.S. Senate from 1980-1993 representing New Hampshire. He worked with George Mitchell on the Iran-Contra and Sharm el-Sheikh committees, but they had known each other earlier through their respective legal careers. He was appointed by President Clinton to the President’s Foreign Intelligence Advisory Board and served from 1993-2001. SummaryInterview includes discussion of: legal careers; Iran-Contra; position of Senate majority leader; Sharm el-Sheikh Fact-Finding Committee; Gramm-Rudman-Hollings Act to balance the budget; relationships between senators; and changes in the Senate over the years.
Interview with Harold Pachios by Andrea L’Hommedieu
Date: 2008-05-12
Creator: Harold 'Hal' Pachios
Access: Open access
- Biographical NoteHarold “Hal” Pachios was born July 12, 1936, in New Haven, Connecticut, and grew up in Cape Elizabeth, Maine. He attended Princeton and Georgetown Law. He served for two years on a Navy transport ship, worked for the Peace Corps as a congressional liaison, then held numerous positions in politics and government including at the Democratic National Committee, VISTA, the White House (as associate White House press secretary), the Department of Transportation, and for Senator Edmund S. Muskie's vice presidential campaign. A long time-friend of Mitchell, at the time of this interview he practiced law at Preti Flaherty Beliveau & Pachios, in Portland, Maine. SummaryInterview includes discussion of: meeting and developing a friendship with George Mitchell; social dinners with Mitchell; tennis and golf as a reflection of Mitchell’s focus and concentration; Mitchell’s parents and their family values; Senator Muskie’s influence; decision to run for chairman of the Democratic National Committee; “throw your hat into the ring”; Mitchell’s 1974 gubernatorial campaign; the boating story; dining with Mitchell; Mitchell’s relationship with his brother Robbie; cribbage; difference between Mitchell and other politicians; Mitchell’s love of sports; and Mitchell’s busy schedule.