ArXiv is a repository of electronic preprints approved for posting after moderation, but not full peer review. It consists of scientific papers in the fields of mathematics, astronomy, electrical engineering, computer science, quantitative biology, mathematical finance and economics, which can be accessed online. In many fields of mathematics and physics all scientific papers are self-archived on the arXiv repository. Begun on August 14, 1991, arXiv.org passed the half-million-article milestone on October 3, 2008, had hit a million by the end of 2014. By October 2016 the submission rate had grown to more than 10,000 per month. ArXiv was made possible by the compact TeX file format, which allowed scientific papers to be transmitted over the Internet and rendered client-side. Around 1990, Joanne Cohn began emailing physics preprints to colleagues as TeX files, but the number of papers being sent soon filled mailboxes to capacity. Paul Ginsparg recognized the need for central storage, in August 1991 he created a central repository mailbox stored at the Los Alamos National Laboratory which could be accessed from any computer.
Additional modes of access were soon added: FTP in 1991, Gopher in 1992, the World Wide Web in 1993. The term e-print was adopted to describe the articles, it began as a physics archive, called the LANL preprint archive, but soon expanded to include astronomy, computer science, quantitative biology and, most statistics. Its original domain name was xxx.lanl.gov. Due to LANL's lack of interest in the expanding technology, in 2001 Ginsparg changed institutions to Cornell University and changed the name of the repository to arXiv.org. It is now hosted principally with eight mirrors around the world, its existence was one of the precipitating factors that led to the current movement in scientific publishing known as open access. Mathematicians and scientists upload their papers to arXiv.org for worldwide access and sometimes for reviews before they are published in peer-reviewed journals. Ginsparg was awarded a MacArthur Fellowship in 2002 for his establishment of arXiv; the annual budget for arXiv is $826,000 for 2013 to 2017, funded jointly by Cornell University Library, the Simons Foundation and annual fee income from member institutions.
This model arose in 2010, when Cornell sought to broaden the financial funding of the project by asking institutions to make annual voluntary contributions based on the amount of download usage by each institution. Each member institution pledges a five-year funding commitment to support arXiv. Based on institutional usage ranking, the annual fees are set in four tiers from $1,000 to $4,400. Cornell's goal is to raise at least $504,000 per year through membership fees generated by 220 institutions. In September 2011, Cornell University Library took overall administrative and financial responsibility for arXiv's operation and development. Ginsparg was quoted in the Chronicle of Higher Education as saying it "was supposed to be a three-hour tour, not a life sentence". However, Ginsparg remains on the arXiv Scientific Advisory Board and on the arXiv Physics Advisory Committee. Although arXiv is not peer reviewed, a collection of moderators for each area review the submissions; the lists of moderators for many sections of arXiv are publicly available, but moderators for most of the physics sections remain unlisted.
Additionally, an "endorsement" system was introduced in 2004 as part of an effort to ensure content is relevant and of interest to current research in the specified disciplines. Under the system, for categories that use it, an author must be endorsed by an established arXiv author before being allowed to submit papers to those categories. Endorsers are not asked to review the paper for errors, but to check whether the paper is appropriate for the intended subject area. New authors from recognized academic institutions receive automatic endorsement, which in practice means that they do not need to deal with the endorsement system at all. However, the endorsement system has attracted criticism for restricting scientific inquiry. A majority of the e-prints are submitted to journals for publication, but some work, including some influential papers, remain purely as e-prints and are never published in a peer-reviewed journal. A well-known example of the latter is an outline of a proof of Thurston's geometrization conjecture, including the Poincaré conjecture as a particular case, uploaded by Grigori Perelman in November 2002.
Perelman appears content to forgo the traditional peer-reviewed journal process, stating: "If anybody is interested in my way of solving the problem, it's all there – let them go and read about it". Despite this non-traditional method of publication, other mathematicians recognized this work by offering the Fields Medal and Clay Mathematics Millennium Prizes to Perelman, both of which he refused. Papers can be submitted in any of several formats, including LaTeX, PDF printed from a word processor other than TeX or LaTeX; the submission is rejected by the arXiv software if generating the final PDF file fails, if any image file is too large, or if the total size of the submission is too large. ArXiv now allows one to store and modify an incomplete submission, only finalize the submission when ready; the time stamp on the article is set. The standard access route is through one of several mirrors. Sev
Silicon is a chemical element with symbol Si and atomic number 14. It is a brittle crystalline solid with a blue-grey metallic lustre, it is a member of group 14 in the periodic table: carbon is above it. It is unreactive; because of its high chemical affinity for oxygen, it was not until 1823 that Jöns Jakob Berzelius was first able to prepare it and characterize it in pure form. Its melting and boiling points of 1414 °C and 3265 °C are the second-highest among all the metalloids and nonmetals, being only surpassed by boron. Silicon is the eighth most common element in the universe by mass, but rarely occurs as the pure element in the Earth's crust, it is most distributed in dusts, sands and planets as various forms of silicon dioxide or silicates. More than 90% of the Earth's crust is composed of silicate minerals, making silicon the second most abundant element in the Earth's crust after oxygen. Most silicon is used commercially without being separated, with little processing of the natural minerals.
Such use includes industrial construction with clays, silica sand, stone. Silicates are used in Portland cement for mortar and stucco, mixed with silica sand and gravel to make concrete for walkways and roads, they are used in whiteware ceramics such as porcelain, in traditional quartz-based soda-lime glass and many other specialty glasses. Silicon compounds such as silicon carbide are used as abrasives and components of high-strength ceramics. Silicon is the basis of the used synthetic polymers called silicones. Elemental silicon has a large impact on the modern world economy. Most free silicon is used in the steel refining, aluminium-casting, fine chemical industries. More visibly, the small portion of highly purified elemental silicon used in semiconductor electronics is essential to integrated circuits – most computers, cell phones, modern technology depend on it. Silicon is an essential element in biology. However, various sea sponges and microorganisms, such as diatoms and radiolaria, secrete skeletal structures made of silica.
Silica is deposited in many plant tissues. In 1787 Antoine Lavoisier suspected that silica might be an oxide of a fundamental chemical element, but the chemical affinity of silicon for oxygen is high enough that he had no means to reduce the oxide and isolate the element. After an attempt to isolate silicon in 1808, Sir Humphry Davy proposed the name "silicium" for silicon, from the Latin silex, silicis for flint, adding the "-ium" ending because he believed it to be a metal. Most other languages use transliterated forms of Davy's name, sometimes adapted to local phonology. A few others use instead a calque of the Latin root. Gay-Lussac and Thénard are thought to have prepared impure amorphous silicon in 1811, through the heating of isolated potassium metal with silicon tetrafluoride, but they did not purify and characterize the product, nor identify it as a new element. Silicon was given its present name in 1817 by Scottish chemist Thomas Thomson, he retained part of Davy's name but added "-on" because he believed that silicon was a nonmetal similar to boron and carbon.
In 1823, Jöns Jacob Berzelius prepared amorphous silicon using the same method as Gay-Lussac, but purifying the product to a brown powder by washing it. As a result, he is given credit for the element's discovery; the same year, Berzelius became the first to prepare silicon tetrachloride. Silicon in its more common crystalline form was not prepared until 31 years by Deville. By electrolyzing a mixture of sodium chloride and aluminium chloride containing 10% silicon, he was able to obtain a impure allotrope of silicon in 1854. More cost-effective methods have been developed to isolate several allotrope forms, the most recent being silicene in 2010. Meanwhile, research on the chemistry of silicon continued; the first organosilicon compound, was synthesised by Charles Friedel and James Crafts in 1863, but detailed characterisation of organosilicon chemistry was only done in the early 20th century by Frederic Kipping. Starting in the 1920s, the work of William Lawrence Bragg on X-ray crystallography elucidated the compositions of the silicates, known from analytical chemistry but had not yet been understood, together with Linus Pauling's development of crystal chemistry and Victor Goldschmidt's development of geochemistry.
The middle of the 20th century saw the development of the chemistry and industrial use of siloxanes and the growing use of silicone polymers and resins. In the late 20th century, the complexity of the crystal chemistry of silicides was mapped, along with the solid-state chemistry of doped semiconductors; because silicon is an important element in high-technology semiconductor devi
Polytechnic University of Milan
The Polytechnic University of Milan is the largest technical university in Italy, with about 42,000 students. It offers undergraduate and higher education courses in engineering and design. Founded in 1863, it is the oldest university in Milan; the Polytechnic University of Milan has two main campuses in the city of Milan, where the majority of the research and teaching activities are located, other satellite campuses in five other cities across Lombardy and Emilia Romagna. The central offices and headquarters are located in the historical campus of Città Studi in Milan, the largest, active since 1927; the university was ranked the best for Engineering and among the top big universities in Italy in the CENSIS-Repubblica Italian University rankings for 2014–15. According to the QS World University Rankings, as of 2018 it is the 17th best technical university in the world, ranking fifth for Design, ninth for Architecture, ninth for Civil and Structural Engineering, 17th for Engineering and Technology.
Its notable alumni include Giulio Natta, Nobel laureate in chemistry in 1963, the two-times nominated novelist Carlo Emilio Gadda. The Polytechnic University of Milan was founded on 29 November 1863 by Francesco Brioschi, secretary of the Ministry of Education and rector of the University of Pavia, it is the oldest university in Milan. Its original name was Istituto Tecnico Superiore and only Civil and Industrial Engineering were taught. Architecture, the second main line of study at the university, was introduced in 1865 in cooperation with the Brera Academy. There were only 30 students admitted in the first year. Over the decades, most of students were men: the first female graduate from the university was in 1913. In 1927 the university moved to piazza Leonardo da Vinci, in the district now known as Città studi, where the university's main facilities are still today. At the time, it was named Regio Politecnico; the word Regio was removed as Italy was proclaimed a republic at the end of World War II.
The historical building still in use today was designed and built by engineers and architects all graduated from the university itself. The present logo, based on a detail of the preparatory sketch of Raphael's School of Athens, was adopted in 1942; until there was no official logo for the institution. In 1954, the first European centre of electronic computation was opened at the university by Gino Cassinis and Ercole Bottani. In 1963 Giulio Natta received the Nobel Prize in Chemistry for his research on crystalline polymers, polypropylene in particular. In 1977, the satellite Sirio, jointly developed by the university and other companies, was launched. Since the end of the 1980s, the university has begun a process of territorial expansion that would have resulted in the opening of its satellite campuses in Lombardy and Emilia Romagna. A university program in industrial design was started in 1993. In 2000, the university's faculty of design was created with new courses in undergraduate and postgraduate programs of graphic & visual and interior design along with the existent industrial design.
In April 2012, the university announced that, beginning in 2014, all graduate courses would be taught only in English. This decision was partially revised, after the decision of the Italian Supreme Court, that stated Italian language could not be abolished nor downgraded to a marginal role; the University is spread over seven campuses: two main campuses in Milan and another five satellite campuses across Lombardy and Emilia Romagna. Milan Leonardo is the oldest of the university's campuses still in use; the first buildings on Piazza Leonardo da Vinci were inaugurated in 1927. Over the years, the complex has been expanded and is now referred to as "Città Studi", City of Studies, which refers to some faculties of the University of Milan in the same area; the campus extends over several streets: Leonardo, Clericetti, Gran Sasso and Colombo. The Leonardo Campus is the main campus of the university, comprises the central administration offices, the rectorate, most of the research departments; the Milan Bovisa campus is located in the Bovisa district of Milan and became active in 1989.
The first is the seat of the School of Design, while the second is dedicated to Industrial, Mechanical and Energy Engineering faculties. Bovisa houses the related research facilities, including the wind tunnel; the first satellite campuses opened in 1987 in 1989 in Lecco. During the 1990s other three branches opened in Cremona and Piacenza; the Polytechnic University of Milan offers several three-year undergraduate courses, two-year graduate courses, one-year master courses and PhD programs in the fields of engineering and design. The university offers 32 first level degree programs; the academic year is divided into two terms, or semesters, the first from mid-September to late January and the second from March to late June. There are 3 exam sessions: those at the end of each semester and one more in September. Students need to achieve 60 "university credits" per year during their Bachelor and master's degrees. Therefore, the 3-years Bachelor requires 180 credits while the 2-years Master 120; the university, like most universities in Italy, is organized to comply with the framework of the Bologna Process.
The university maintains several relations with foreign universities and offers a wide range of international projects for student exchange, The university enc
Yale University is a private Ivy League research university in New Haven, Connecticut. Founded in 1701, it is the third-oldest institution of higher education in the United States and one of the nine Colonial Colleges chartered before the American Revolution. Chartered by Connecticut Colony, the "Collegiate School" was established by clergy to educate Congregational ministers, it moved to New Haven in 1716 and shortly after was renamed Yale College in recognition of a gift from British East India Company governor Elihu Yale. Restricted to theology and sacred languages, the curriculum began to incorporate humanities and sciences by the time of the American Revolution. In the 19th century, the college expanded into graduate and professional instruction, awarding the first Ph. D. in the United States in 1861 and organizing as a university in 1887. Its faculty and student populations grew after 1890 with rapid expansion of the physical campus and scientific research. Yale is organized into fourteen constituent schools: the original undergraduate college, the Yale Graduate School of Arts and Sciences and twelve professional schools.
While the university is governed by the Yale Corporation, each school's faculty oversees its curriculum and degree programs. In addition to a central campus in downtown New Haven, the university owns athletic facilities in western New Haven, a campus in West Haven and forest and nature preserves throughout New England; the university's assets include an endowment valued at $29.4 billion as of October 2018, the second largest endowment of any educational institution in the world. The Yale University Library, serving all constituent schools, holds more than 15 million volumes and is the third-largest academic library in the United States. Yale College undergraduates follow a liberal arts curriculum with departmental majors and are organized into a social system of residential colleges. All members of the Faculty of Arts and Sciences—and some members of other faculties—teach undergraduate courses, more than 2,000 of which are offered annually. Students compete intercollegiately as the Yale Bulldogs in the NCAA Division I – Ivy League.
As of October 2018, 61 Nobel laureates, 5 Fields Medalists and 3 Turing award winners have been affiliated with Yale University. In addition, Yale has graduated many notable alumni, including five U. S. Presidents, 19 U. S. Supreme Court Justices, 31 living billionaires and many heads of state. Hundreds of members of Congress and many U. S. diplomats, 78 MacArthur Fellows, 247 Rhodes Scholars and 119 Marshall Scholars have been affiliated with the university. Its wealth and influence have led to Yale being reported as amoungst the most prestigious universities in the United States. Yale traces its beginnings to "An Act for Liberty to Erect a Collegiate School", passed by the General Court of the Colony of Connecticut on October 9, 1701, while meeting in New Haven; the Act was an effort to create an institution to train ministers and lay leadership for Connecticut. Soon thereafter, a group of ten Congregational ministers, Samuel Andrew, Thomas Buckingham, Israel Chauncy, Samuel Mather, Rev. James Noyes II, James Pierpont, Abraham Pierson, Noadiah Russell, Joseph Webb, Timothy Woodbridge, all alumni of Harvard, met in the study of Reverend Samuel Russell in Branford, Connecticut, to pool their books to form the school's library.
The group, led by James Pierpont, is now known as "The Founders". Known as the "Collegiate School", the institution opened in the home of its first rector, Abraham Pierson, today considered the first president of Yale. Pierson lived in Killingworth; the school moved to Saybrook and Wethersfield. In 1716, it moved to Connecticut. Meanwhile, there was a rift forming at Harvard between its sixth president, Increase Mather, the rest of the Harvard clergy, whom Mather viewed as liberal, ecclesiastically lax, overly broad in Church polity; the feud caused the Mathers to champion the success of the Collegiate School in the hope that it would maintain the Puritan religious orthodoxy in a way that Harvard had not. In 1718, at the behest of either Rector Samuel Andrew or the colony's Governor Gurdon Saltonstall, Cotton Mather contacted the successful Boston born businessman Elihu Yale to ask him for financial help in constructing a new building for the college. Through the persuasion of Jeremiah Dummer, Elihu "Eli" Yale, who had made a fortune through trade while living in Madras as a representative of the East India Company, donated nine bales of goods, which were sold for more than £560, a substantial sum at the time.
Cotton Mather suggested that the school change its name to "Yale College".. Meanwhile, a Harvard graduate working in England convinced some 180 prominent intellectuals that they should donate books to Yale; the 1714 shipment of 500 books represented the best of modern English literature, science and theology. It had a profound effect on intellectuals at Yale. Undergraduate Jonathan Edwards discovered John Locke's works and developed his original theology known as the "new divinity". In 1722 the Rector and six of his friends, who had a study group to discuss the new ideas, announced that they had given up Calvinism, become Arminians and joined the Church of England, they were returned to the colonies as missionaries for the Anglican faith. Thomas Clapp became president in 1745 and struggled to return the college to Calvinist orthodoxy, but he did not close the library. Other students found Deist books in the library. Yale was swept up by the great intellectual movements of the peri
Active laser medium
The active laser medium is the source of optical gain within a laser. The gain results from the stimulated emission of electronic or molecular transitions to a lower energy state from a higher energy state populated by a pump source. Examples of active laser media include: Certain crystals doped with rare-earth ions or transition metal ions. Liquids, in the form of dye solutions as used in dye lasers. In order to fire a laser, the active gain medium must be in a nonthermal energy distribution known as a population inversion; the preparation of this state is known as laser pumping. Pumping may be achieved with electrical currents or with light, generated by discharge lamps or by other lasers. More exotic gain media can be pumped by chemical reactions, nuclear fission, or with high-energy electron beams. A universal model valid for all laser types does not exist; the simplest model includes two systems of sub-levels: lower. Within each sub-level system, the fast transitions ensure that thermal equilibrium is reached leading to the Maxwell–Boltzmann statistics of excitations among sub-levels in each system.
The upper level is assumed to be metastable. Gain and refractive index are assumed independent of a particular way of excitation. For good performance of the gain medium, the separation between sub-levels should be larger than working temperature. In the case of amplification of optical signals, the lasing frequency is called signal frequency. However, the same term is used in the laser oscillators, when amplified radiation is used to transfer energy rather than information; the model below seems to work well for most optically-pumped solid-state lasers. The simple medium can be characterized with effective cross-sections of absorption and emission at frequencies ω p and ω s. Have N be concentration of active centers in the solid-state lasers. Have N 1 be concentration of active centers in the ground state. Have N 2 be concentration of excited centers. Have N 1 + N 2 = N; the relative concentrations can be defined as n 1 = N 1 / N and n 2 = N 2 / N. The rate of transitions of an active center from ground state to the excited state can be expressed with W u = I p σ a p ℏ ω p + I s σ a s ℏ ω s and The rate of transitions back to the ground state can be expressed with W d = I p σ e p ℏ ω p + I s σ e s ℏ ω s + 1 τ, where σ a s and σ a p are effective cross-sections of absorption at the frequencies of the signal and the pump.
Σ e s and σ e p are the same for stimulated emission. The kinetic equation for relative populations can be written as follows: d n 2 d t = W u n 1 − W d n 2, d n 1 d t = − W