AcharyaChanakya/simple-wikipedia huggingface dataset:
Mobile Instant Messaging (MIM) is a presence enabled messaging service that aims to transpose the desktop messaging experience to the usage scenario of being on the move. While several of the core ideas of the desktop experience on one hand apply to a connected mobile device, others do not: Users usually only look at their phone's screen — presence status changes might occur under different circumstances as happens at the desktop, and several functional limits exist based on the fact that the vast majority of mobile communication devices are chosen by their users to fit into the palm of their hand.
Some of the form factor and mobility related differences need to be taken into account in order to create a really adequate, powerful and yet convenient mobile experience: radio bandwidth, memory size, availability of media formats, keypad based input, screen output, CPU performance and battery power are core issues that desktop device users and even nomadic users with connected notebooks are usually not exposed to.
Several formerly untackled issues have been identified and addressed within IMPS, which was developed as part of an early mobile telephone industry initiative to kick off a broader usage of mobile instant messaging. The Open Mobile Alliance has taken over this standard, formerly called Wireless Village, as IMPS V1.0 in November 2002. Since then this standards has been further developed to IMPS V1.3, the latest candidate for release, and is expected to be released before the end of 2006.
There are downloadable mobile applications offered by different independent developers that allow users to chat within public (MSN, Yahoo!, Google Talk, AIM, ICQ) and corporate (LCS, Sametime, Reuters) IM services from mobile devices.
Among the advantages of using such IM clients over SMS are: IM clients use data instead of SMS text messages; IM-like chat mode, faster and quicker messaging. Some IM software allows group communication.
Several large scale mobile telephone industry companies are planning to jointly deliver a ubiquitous, interoperable presence enabled messaging service, built according to interoperability recommendations developed in the GSM Association. Because these organisations, as a group, have about 1.5 billion active Short Text Messaging (SMS) users, people wonder if such an initiative might also help the different industry factions to begin to work together soon. This would make Mobile Instant Messaging similar for all users.
In the meantime, other developments have proposed usage of downloadable applications with the intention to create their own approach to IM that runs on most mobile phones worldwide. Essentially, several of these clients are Java applications are instantly downloaded and then connected to back-end servers through GPRS/3G Internet Channels. Some of the implementations can connect to other IM services.
Instant messaging opens new methods of spontaneous communication for people that have an impairment in hearing, auditory processing, or speech. It is considered by many a powerful way to allow equal opportunities in communication, without the aid of special devices or services designed for users with hearing loss.
Instant Messaging may be done in a Friend-to-friend network, in which each node connects to the friends on the friendslist. This allows to communicate to friends of friends and build chatrooms for instant messages with all friends on that network.
Instant messaging has proven to be similar to personal computers, e-mail, and the WWW, in that its adoption for use as a business communications medium was driven primarily by individual employees using consumer software at work, rather than by formal mandate or provisioning by corporate information technology departments. Tens of millions of the consumer IM accounts in use are being used for business purposes by employees of companies and other organizations.
In response to the demand for business-grade IM and the need to ensure security and legal compliance, a new type of instant messaging, called "Enterprise Instant Messaging" ("EIM") was created when Lotus Software launched Lotus Sametime in 1999. Microsoft followed suit shortly thereafter with Microsoft Exchange Instant Messaging, and later created a new platform called Microsoft Office Live Communications Server. Since then, both IBM Lotus and Microsoft have introduced federation between their EIM systems and some of the public IM networks thus employees may use a single interface to both their internal EIM system and their buddies on AOL, MSN, and Yahoo!. Current leading EIM platforms include IBM Lotus Sametime, Microsoft Office Live Communications Server, and Jabber XCP .
The adoption of IM across corporate networks outside of the control of IT organizations creates many risks and liabilities for companies who do not effectively manage and support IM use. Companies implement specialized IM archiving and security products and services like those from Secure Computing, Akonix, Surfcontrol, and ScanSafe to mitigate these risks and provide safe, secure, productive instant messaging capabilities to their employees.
Although instant messaging has many good things, it also has risks and liabilities, especially when used in "workplaces". Among these are:
Hackers' use of instant messaging networks to deliver malicious code has grown consistently from 2004 to the present, with the number of discrete attacks listed by the IM Security Center having grown 15% from 347 attacks in 2005 to 406 in 2006. Hackers use two methods of delivering malicious code through IM: delivery of virus, trojan, or spyware within an infected file, and the use of "socially engineered" text with a web address that entices the recipient to click on a URL that connects him or her to a website that then downloads malicious code. Viruses, worms, and trojans typically propagate by sending themselves rapidly through the infected user's buddy list. An effective attack using a "poison URL" may reach tens of thousands of people in minutes when each person's buddy list receives messages appearing to be from a trusted friend. The recipients click on the web address, and the entire cycle starts again. Infections may range from nuisance to criminal, and are becoming more sophisticated each year.
In addition to the malicious code threat, the use of instant messaging at work also creates a risk of non-compliance to laws and regulations governing the use of electronic communications in businesses. In the United States alone there are over 10,000 laws and regulations related to electronic messaging and records retention. The more well-known of these include the Sarbanes-Oxley Act, HIPAA, and SEC 17a-3. Recent changes to Federal Rules of Civil Procedure, effective December 1, 2006, create a new category for electronic records which may be requested during discovery (law) in legal proceedings. Most countries around the world also regulate the use of electronic messaging and electronic records retention in similar fashion to the United States. The most common regulations related to IM at work involve the need to produce archived business communications to satisfy government or judicial requests under law. Many instant messaging communications fall into the category of business communications that must be archived and retrievable.
Organizations of all types must protect themselves from the liability of their employees' inappropriate use of IM. The informal, immediate, and ostensibly anonymous nature of instant messaging makes it a candidate for abuse in the workplace. The topic of inappropriate IM use became front page news in October 2006 when Congressman Mark Foley resigned his seat after admitting sending offensive instant messages of a sexual nature to underage former House pages from his Congressional office PC. The Mark Foley Scandal led to media coverage and mainstream newspaper articles warning of the risks of inappropriate IM use in workplaces. In most countries, corporations have a legal responsibility to ensure harassment-free work environment for employees. The use of corporate-owned computers, networks, and software to harass an individual or spread inappropriate jokes or language creates a liability for not only the offender but also the employer. A survey by IM archiving and security provider Akonix Systems, Inc. in March 2007 showed that 31% of respondents had been harassed over IM at work. Companies now include instant messaging as an integral component of their policies on appropriate use of the World Wide Web, email, and other corporate assets.
In the early 2000s, a new class of IT security provider emerged to provide remedies for the risks and liabilities faced by corporations who chose to use IM for business communications. The IM security providers created new products to be installed in corporate networks for the purpose of archiving, content-scanning, and security-scanning IM traffic moving in and out of the corporation. Similar to the e-mail filtering vendors, the IM security providers focus on the risks and liabilities described above.
With rapid adoption of IM in the workplace, demand for IM security products began to grow in the mid-2000s. By 2007, the preferred platform for the purchase of security software had become the "appliance", according to IDC, who estimate that by 2008, 80% of network security products will be delivered via an appliance.
AOL Inc. (previously America Online) is a U.S.-based online service provider and Internet service provider. It has been in business since 1985. Important services in the early years were online chat and online games. Most connections were dial-up. In 2001 AOL bought Time Warner. In 2009 after a spin-off from Time Warner they changed their name to AOL Inc. It was announced in May 2015 that Verizon Communications purchased AOL.
AOL Instant Messenger was a service which allowed users to communicate with any user currently online. AIM had millions of users worldwide. There were also other computer programs that could be used to talk with your AIM account, for example Trillian, Pidgin, Digsby, and AIM+. AIM was shut down on December 15, 2017.
Other AOL products are AOL Spyware Protection, AOL Safety and Security Center, ICQ, AOl Computer Check-Up, AOL Explorer, AOL TopSpeed, AOL Dialer, AOL One-Click Fixes and AOL Openride.
Youssef Rzouga
Youssef Rzouga (born March 21, 1957) is a Tunisian poet. He was born in Mahdia, a Tunisian coastal city.
His first writing to be printed was a short story in the magazine "Radio et Télévision", in 1973.
A mailman or mail carrier or postman is a person who brings people their postal mail. Normally he works for the post office of the respective country. Of course, there are also women doing this job. They are called mailwomen. For a layman, this occupation is called "letter carriers".
Fictional mailmen are traditionally shown as acting afraid of dogs, even to the point of shock.
The Chinese postman problem is a mathematical problem of graph theory. It is also known as . Suppose there is a mailman who needs to deliver mail to a certain neighbourhood. The mailman is unwilling to walk far, so he wants to find the shortest route through the neighbourhood, that meets the following criteria:
If the graph travelled has an Eulerian path, this circuit is the ideal solution.
Alan Goldman of the U.S. National Bureau of Standards first coined the name 'Chinese Postman Problem' for this problem, as it was originally studied by the Chinese mathematician Mei-Ku Kuan in 1962.
The four color theorem is a theorem of mathematics. It says that in any plane surface with regions in it (people think of them as maps), the regions can be colored with no more than four colors. Two regions that have a common border must not get the same color. They are called "adjacent" (next to each other) if they share a segment of the border, not just a point.
This was the first theorem to be proved by a computer, in a "proof by exhaustion". In proof by exhaustion, the conclusion is established by dividing it into cases, and proving each one separately. There may be a lot of cases. For example, the first proof of the four color theorem was a proof by exhaustion with 1,936 cases. This proof was controversial because most of the cases were checked by a computer program, not by hand. The shortest known proof of the four color theorem today still has over 600 cases.
Even though the problem was first presented as a problem to color political maps of countries, mapmakers are not very interested in it. According to an article by the math historian Kenneth May , “Maps utilizing only four colors are rare, and those that do usually require only three. Books on cartography and the history of map making do not mention the four-color property.”
Many simpler maps can be colored using three colors. The fourth color is needed for some maps, such as one in which one region is surrounded by an odd number of others, which touch each other in a cycle. One such example is given in the image. The five color theorem states that five colors are enough to color a map. It has a short, elementary proof and was proven in the late 19th century. Proving that four colors are all that is needed turned out to be much more difficult. Many false proofs and false counterexamples have appeared since the first statement of the four color theorem in 1852.
Intuitively, the four color theorem can be stated as 'given any separation of a plane into contiguous regions, called a map, the regions can be colored using at most four colors so that no two regions which are adjacent have the same color'. To be able to correctly solve the problem, it is necessary to clarify some aspects: First, all points that belong to three or more countries must be ignored. Secondly, "bizarre" maps with regions of finite area and infinite perimeter can require more than four colors.
For the purpose of the theorem every "country" has to be a simply connected region, or contiguous.
In the real world, this is not true: Alaska as part of the United States, Nakhchivan as part of Azerbaijan, and Kaliningrad as part of Russia are not contiguous. Because the territory of a particular country must be the same color, four colors may not be enough. For instance, consider a simplified map, such as the one shown on the left: In this map, the two regions labeled "A" belong to the same country, and must be the same color. This map then requires five colors, since the two "A" regions together are contiguous with four other regions, each of which is contiguous with all the others. If "A" had only three regions, six or more colors might be needed. In this way, it is possible to make maps that need an arbitrarily high number of colors. A similar construction also applies if a single color is used for all bodies of water, as is usual on real maps.
An easier to state version of the theorem uses graph theory. The set of regions of a map can be represented more abstractly as an undirected graph that has a vertex for each region and an edge for every pair of regions that share a boundary segment. This graph is planar: it can be drawn in the plane without crossings by placing each vertex at an arbitrarily chosen location within the region to which it corresponds, and by drawing the edges as curves that lead without crossing within each region from the vertex location to each shared boundary point of the region. Conversely any planar graph can be formed from a map in this way. In graph-theoretic terminology, the four-color theorem states that the vertices of every planar graph can be colored with at most four colors so that no two adjacent vertices have the same color, or for short, "every planar graph is four-colorable" (; ).
The first person to name the problem was Francis Guthrie, in 1852. He was a law student in England, at the time. He found that he needed at least four colors to color a map of the counties of England. Augustus de Morgan first discussed the problem, in a letter he wrote to Rowan Hamliton in August 1852. In the letter, de Morgan asks whether four colors are really enough to color a map, such that countries that are next to each other get different colors.
English mathematician Arthur Cayley presented the problem to the mathematical society in London, in 1878. Within a year, Alfred Kempe found what looked like a proof of the problem. Eleven years later, in 1890, Percy Heawood showed that Alfred's proof was wrong. Peter Guthrie Tait presented another attempt at a proof in 1880. It took eleven years to show that Tait's proof did not work either. In 1891, Julius Petersen could show this. When he falsified Cayley's proof, Kempe also showed a proof for a problem he called Five color theorem. The theorem says that any such map can be colored with no more than five colors. There are two restrictions: First, any country is contiguous, there are no exclaves. The second restriction is that countries need to have a common border; if they only touch in a point, they can be colored with the same color. Even though Kempe's proof was wrong, he used some of the ideas which would later permit a correct proof.
In the 1960s and 1970s, Heinrich Heesch developed a first sketch of a proof by computer. Kenneth Appel and Wolfgang Haken improved this sketch in 1976 . They were able to reduce the number of cases that would need to be tested to 1936; a later version was made that relied on testing only 1476 cases. Each case needed to be tested by a computer .
In 1996, Neil Robertson, Daniel Sanders, Paul Seymour and Robin Thomas improved the method, and reduced the number of cases to be tested to 663. Again, each case needed to be tested by computer.
In 2005, Georges Gonthier and Benjamin Werner developed a formal proof. This was an improvement, because it allowed to use theorem-proving software, for the first time. The software used is called Coq.
The four color theorem is the first big mathematical problem that was proved with the help of a computer. Because the proof cannot be done by a human, some mathematicians did not recognize it as correct. To verify the proof, it is necessary to rely on a correctly working software and hardware to validate the proof. Because the proof was done by computer, it is also not very elegant.
The four color theorem has been notorious for attracting a large number of false proofs and disproofs in its long history. At first, "The New York Times" refused to report on the Appel–Haken proof. The newspaper did this as a matter of policy; it feared that the proof would be shown false like the ones before it .
Some proofs took a long time, till they could be falsified: For Kempe's and Tait's proofs falsifying them took over a decade.
The simplest counterexamples generally try to create one region which touches all the others. This forces the remaining regions to be colored with only three colors. Because the four color theorem is true, this is always possible; however, because the person drawing the map is focused on the one large region, they do not notice that the remaining regions can in fact be colored with three colors.
This trick can be generalized: If the colors of some regions in a map are chosen beforehand, it becomes impossible to color the remaining regions in such a way that in total, only four colors are used. Someone verifying the counterexample may not think that it may be needed to change the color of these regions. This will make the counterexample look valid, even though it is not.
Perhaps one effect underlying this common misconception is the fact that the color restriction is not transitive: a region only has to be colored differently from regions it touches directly, not regions touching regions that it touches. If this were the restriction, planar graphs would require arbitrarily large numbers of colors.
Other false disproofs violate the assumptions of the theorem in unexpected ways, such as using a region that has multiple disconnected parts, or not allowing regions of the same color from touching at a point.
In real life, many countries have exclaves or colonies. Since they belong to the country, they need to be colored with the same color as the parent country. This means that usually, more than four colors are needed to color such a map. When mathematicians talk about the graph associated with the problem, they say that is not planar. Even though it is easy to check if a graph is planar, finding the fewest number of colors needed to color it is very difficult. It is NP-complete, which is one of the most difficult problems that exist. The fewest number of colors needed to color a graph is known as its "chromatic number". Many of the problems that happen when trying to solve the four color theorem are related to discrete mathematics. For this reason, methods from algebraic topology are often used.
The four color theorem requires the "map" to be on a flat surface, what mathematicians call a plane. In 1890, Percy John Heawood created what is called Heawood conjecture today: It asks the same question as the four color theorem, but for any topological object. As an example, a torus can be colored with at most seven colors. The Heawood conjecture gives a formula that works for all such objects, except the Klein bottle.
A theorem is a proven idea in mathematics. Theorems are proved using logic and other theorems that have already been proved. A minor theorem that one must prove to prove a major theorem is called a lemma. Theorems are made of two parts: hypotheses and conclusions.
Theorems use deduction, in contrast to theories which are empirical.
Some theorems are trivial, since they directly follow from the propositions. Other theorems are called "deep", because their proof is long and difficult. Sometimes, such proofs involve other areas of mathematics or show connections between different areas. A theorem might be simple to state and yet be deep. An excellent example is Fermat's Last Theorem, and there are many other examples of simple yet deep theorems in number theory and combinatorics (among other areas).
There are other theorems for which a proof is known, but cannot be easily written down. Among the best examples are the four color theorem and the Kepler conjecture. Both of these theorems are only known to be true by reducing them to a computational search which is then verified by a computer program. At first, many mathematicians did not accept this form of proof, but it has become more widely accepted over the years. The mathematician Doron Zeilberger has even gone so far as to claim that these are possibly the only nontrivial results that mathematicians have ever proved. Many mathematical theorems can be reduced to simpler computations, including polynomial identities, trigonometric identities and hypergeometric identities.
Triceratops was a huge herbivorous ceratopsid dinosaur from the late Cretaceous. Its name came from having three horns on its head. They were mainly found in North America. As adults, they grew up to 30 feet long by 9 feet tall (), and probably weighed around 5,400 kg (12,000 lb).
"Triceratops" was a low browser with a bony beak in front of its jaws. The jaws had close-set grinding teeth. Its defence had to stand up to attacks from taller theropods, hence the bony shield which covered its neck. Holes made by teeth have been found on the bony frill behind the horns, and on the sacrum (the part of the spine above the pelvis).
Many fossils of "Triceratops" have been collected since the genus was first described in 1889. There is at least one complete individual skeleton. Paleontologist John Scannella observed: "It is hard to walk out into the Hell Creek Formation and not stumble upon a "Triceratops" weathering out of a hillside". Forty-seven complete or partial skulls were discovered in just that area during the decade 2000–2010. Specimens showing life stages from hatchling to adult have been found.
Individual "Triceratops" were about 7.9 to 9.0 m (26.0–29.5 ft) in length, 2.9 to 3.0 m (9.5–9.8 ft) in height, and 6.1–12.0 tonnes (13,000–26,000 lb) in weight.
The most distinctive feature is their large skull, among the largest of all land animals. The largest known skull (specimen BYU 12183) is estimated to have been in length when complete, and could reach almost a third of the length of the entire animal. It bore a single horn on the snout, above the nostrils, and a pair of horns approximately 1 m (3 ft) long, with one above each eye. Most other ceratopsids had large holes (fenestrae) in their frills, while those of "Triceratops" were noticeably solid.
"Triceratops" species were sturdy, with strong limbs and short three-hoofed hands and four-hoofed feet.
The posture of these dinosaurs has long been the subject of some debate. Originally, it was believed that the front legs of the animal had to be sprawling at angles from the thorax, in order to better bear the weight of the head. This stance can be seen in paintings by Charles Knight and Rudolph Zallinger. However, evidence of trackways, and reconstructions of skeletons show that "Triceratops" and other ceratopsids had an upright stance during normal locomotion, with the elbows flexed and slightly bowed out. The stance was intermediate between fully upright and fully sprawling, similar the modern rhinoceros.
Although "Triceratops" are commonly portrayed as herd animals, there is little evidence that they lived in herds.
In 2012, a group of three "Triceratops" in relatively complete condition, each of varying sizes from a full-grown adult to a small juvenile, were found in Wyoming, near Newcastle. The remains are currently under excavation by paleontologist Peter Larson and a team from the Black Hills Institute. It is believed that the animals were traveling as a family unit, but it remains unknown if the group consists of a mated pair and their offspring, or two females and a juvenile they were caring for. The remains also show signs of predation or scavenging from "Tyrannosaurus", particularly on the largest specimen, with the bones of the front limbs showing breakage and puncture wounds from "Tyrannosaurus" teeth.
For many years, "Triceratops" finds were known only from solitary individuals. These remains are very common: one palaeontologist reported having seen 200 specimens of "T. prorsus" in the Hell Creek Formation of Montana, U.S. Similarly, Barnum Brown claimed to have seen over 500 skulls in the field. "Triceratops" teeth, horn fragments, frill fragments, and other skull fragments are abundant fossils in the latest Upper Cretaceous of western North America. It was the most dominant herbivore of the time. In 1986, Robert Bakker estimated it as making up 5/6ths of the large dinosaur fauna at the end of the Cretaceous.
"Triceratops" was one of the last ceratopsian genera to appear before the Cretaceous–Paleogene extinction event. The related "Torosaurus", and the more distantly related diminutive "Leptoceratops", were also present, though their remains are rarely found.
"Triceratops" were herbivorous, and because of their low head, their primary food was probably low growth, although they may have been able to knock down taller plants with their horns, beak, and bulk. The jaws were tipped with a deep, narrow beak, good for grasping and plucking.
"Triceratops" teeth were arranged in groups called batteries, of 36 to 40 tooth columns, in each side of each jaw with 3 to 5 stacked teeth per column, depending on the size of the animal. This gives a range of 432 to 800 teeth, of which only a fraction were in use at any given time (tooth replacement was continuous and occurred throughout the life of the animal). The great size and numerous teeth of "Triceratops" suggests that they ate large volumes of fibrous plant material, such as palms and cycads.
There has been much speculation over the functions of "Triceratops"' head adornments. The two main theories have revolved around use in combat, or display in courtship, with the latter thought now to be the most likely primary function.
"Triceratops" were long thought to have used their horns and frills in combat with predators such as "Tyrannosaurus". The idea was discussed first in 1917 and again 70 years later by Robert Bakker. There is evidence that "Tyrannosaurus" did have aggressive head-on encounters with "Triceratops", based on partially healed tyrannosaur tooth marks on a "Triceratops" brow horn and squamosal; the bitten horn is also broken, with new bone growth after the break. Since the "Triceratops" wounds healed, the "Triceratops" survived the encounter. "Tyrannosaurus" is also known to have fed on "Triceratops". Evidence for this includes a heavily tooth-scored "Triceratops" ilium and sacrum.
In addition to combat with predators using horns, "Triceratops" are classically shown engaging each other in combat with horns locked. While studies show that such activity would be feasible, if unlike that of present-day horned animals, there is disagreement about whether they did so.
The large frill also may have helped to increase body area to regulate body temperature. A similar theory has been proposed regarding the plates of "Stegosaurus", although this use alone would not account for the bizarre and extravagant variation seen in different members of the Ceratopsidae. This observation is highly suggestive of what is now believed to be the primary function, display.
The theory of their use in sexual display was first proposed by Davitashvili in 1961 and has gained increasing acceptance since. Evidence that visual display was important, either in courtship or in other social behavior, can be seen in the fact that horned dinosaurs differ markedly in their adornments, making each species highly distinctive. Also, modern living creatures with such displays of horns and adornments use them in similar behavior. A 2006 study of the smallest "Triceratops" skull, ascertained to be a juvenile, shows the frill and horns developed at a very early age, predating sexual development and thus probably important for visual communication and species recognition in general.
One skull, assigned to "Triceratops", has a hole in the jugal bone. It looks like a puncture wound sustained while the animal was still alive. This is supported by signs of healing that are present in the bone around the supposed wound. When examined closely, the hole in the bone has a diameter that is very similar to diameter of the distal end of a "Triceratops" horn. This is evidence of competition between individual dinosaurs.
A genus is a rank in the biological classification (or taxonomy). It is above species, and below families. A genus can include more than one species. When biologists talk about a genus, they mean one or more species of animals or plants that are closely related to each other.
As with other taxa, the plural is different from other English words because it is a Latin word. 'Genus' is the singular, and 'genera' is the plural form of the word.
When printing the scientific name of an organism, the name is "always in italic". A name of species has two parts, with the genus first. For example, in ""Felis silvestris"", "Felis" is the genus. The genus name always begins with a capital letter. In ""Felis silvestris catus"", the third word is the subspecies, which is not often used.
In writing, genus names in Latin may be 'anglicised' to form a common name. For example, the genus "Pseudomonas" is "pseudomonad" (plural: "pseudomonads"). In practice, most really common animals and plants already have a common name. So instead of saying 'felids', or 'felines', one says 'cats' both for the family pet, and for all the cat family (Felidae).
Cretaceous
The Cretaceous period is the ninth and longest period in the Phanerozoic eon, lasting 80 million years from 145 to 65 million years ago (mya). It followed the Jurassic period, and is the third and last period of the Mesozoic era. After it came the Cainozoic era.
Though famous for its land fauna, the Cretaceous sea level rose much higher than today. North America was partly covered by an epicontinental inland sea, and half of Britain was under water.
In the Upper Cretaceous, chalk, a type of limestone, was laid down in warm shallow seas. Chalk is 95% to 99% made up of coccoliths, the calcium carbonate (CaCO3) plates which tiny single-celled algae (coccolithophorids) produce.
At its end came the famous meteorite strike which, with the volcanic flood basalts spewed out in the Deccan Plateau (India), ended so many of the dominant life-forms: see K/T extinction event.
The Cretaceous period has just two very long epochs:
The Lower Cretaceous at ~45 million years, is the longest epoch in the Phanerozoic eon.
A textbook is a book used for the study of a subject. People use a textbook to learn facts and methods about a certain subject. Textbooks sometimes have questions to test the knowledge and understanding of the learner.
A workbook is a type of textbook that has only practice questions and exercises. Workbooks are designed not to teach but to provide practice and to highlight areas which need more learning. A revision guide is a type of textbook that is used to remind the learner about the subject and give him/her extra practice, especially before an examination.
A textbook is usually lent to students by a school to accompany a course the school is teaching. Sometimes, especially at university, students have to buy the textbooks they need themselves or borrow them from a library.
Most textbooks are only published in printed format. However, some are now available online as electronic books.
An axiom is a concept in logic. It is a statement which is assumed to be true without question, and which does not require proof. It is also known as a postulate (as in the parallel postulate). The axiom is to be used as the premise or starting point for further reasoning or arguments, usually in logic or in mathematics.
This means it cannot be proved within the discussion of a problem. So inside some discussion, it is thought to be true. There are many reasons why it has no proof. For example,
Logic can be used to find theorems from the axioms. Then those theorems can be used to make more theorems. This is often how math works. Axioms are important because logical arguments start with them.
Euclid of Alexandria was a Greek mathematician. Around the year 300 BC, he made the earliest list of axioms which we know of. The following are just a few of them:
A set is an idea from mathematics. A set has "members" (also called "elements"). A set is defined by its members, so any two sets with the same members are the same (e.g., if set formula_1 and set formula_2 have the same members, then "formula_3").
A set cannot have the same member more than once. Membership is the only thing that matters. For example, there is no order or other difference among the members. "Anything" can be a member of a set, including sets themselves (though if a set is a member of itself, paradoxes such as Russell's paradox can happen).
Imagine the set is a bag.
Various things can be put into a bag. Later on, a good question would be if a certain thing is in the bag. Mathematicians call this "element of". Something is an element of a set, if that thing can be found in the respective bag. The symbol used for this is formula_4:
formula_5,
which means that formula_6 is in the bag formula_7 or formula_6 is an element of formula_7.
Unlike a bag, a set can contain at most one item of a given type. So for a set of fruits, it would make no difference if there is one orange, or if there are 10 oranges.
Like a bag, a set can also be "empty". The empty set is like an empty bag: it has nothing in it. The "empty set" is also called the "null set" and is represented by the symbol formula_10.
If we consider, say, some sets of American cars, e.g. a set of all Fords and a set of all Dodges, we may also wish to consider the whole set of American cars. In this case, the set of all American cars would be called a universe.
In other words, a universe is a collection of all the elements one wishes to consider in a given problem. The universe is usually named formula_11.
Two sets can be compared. This is like looking into two different bags. If they contain the same things, they are equal. No matter, in which order these things are.
"For example", if formula_12 and formula_13, the sets are the same.
When mathematicians talk about a set, they sometimes want to know how big a set is (or what is the cardinality of the set). They do this by counting how many elements are in the set (how many items are in the bag). For finite sets the cardinality is a simple number. The empty set has a cardinality of 0. The set formula_14 has a cardinality of 2.
Two sets have the same cardinality if we can pair up their elements—if we can join two elements, one from each set. The set formula_14 and the set formula_16 have the same cardinality. E.g, we could pair "apple" with "sun", and "orange" with "moon". The order does not matter. It is possible to pair all the elements, and none is left out. But the set formula_17 and the set formula_18 have different cardinality.
If we try to pair them up, we always leave out one animal.
At times cardinality is not a number. Sometimes a set has infinite cardinality. The set of all integers is a set with infinite cardinality. Some sets with infinite cardinality are bigger (have a bigger cardinality) than others. There are more real numbers than there are natural numbers, for example, which means we cannot pair up the set of integers and the set of real numbers, even if we worked forever.
If you can count the elements of a set, it is called a countable set. Countable sets include all sets with a finite number of members. Countable sets also include some infinite sets, such as the natural numbers. You can count the natural numbers with formula_19. The natural numbers are nicknamed "the counting numbers", since they are what we usually use to count things with.
An uncountable set is an infinite set that is impossible to count. If we try to count the elements, we will always skip some. It does not matter what step we take. The set of real numbers is an uncountable set. There are many other uncountable sets, even such a small interval like formula_20.[3]
If you look at the set formula_21 and the set formula_22, you can see that all elements in the first set are also in the second set.<br> We say: formula_23 is a subset of formula_24<br>
As a formula it looks like this:<br>
In general, when all elements of set formula_26 are also elements of set formula_27, we call formula_26 a subset of formula_27:<br>
formula_30.<br>
It is usually read "formula_26 is contained in formula_27."
"Example:" Every Chevrolet is an American car. So the set of all Chevrolets is contained in the set of all American cars.
There are different ways to combine sets.
The intersection formula_33 of two sets formula_26 and formula_27 is a set that contains all the elements that are both in set formula_26 and in set formula_27 at the same time.<br>
"Example:" When formula_26 is the set of all cheap cars, and formula_27 is the set of all American cars, then formula_33 is the set of all cheap American cars.
The union formula_41 of two sets formula_26 and formula_27 is a set that contains all the elements that are in set formula_26 or in set formula_27.
This "or" is the "inclusive" disjunction, so the union also contains the elements, that are in set formula_26 "and" in set formula_27.
By the way, this means, that the intersection is a subset of the union:
formula_48.
"Example:" When formula_26 is the set of all cheap cars, and formula_27 is the set of all American cars, then formula_41 is the set of all cars, without all expensive cars that are not from America.
Complement can mean two different things:
formula_55 <br>
The universe formula_11 is the set of all things you speak about.<br>
"Example:" When formula_11 is the set of all cars, and formula_26 is the set of all cheap cars,<br> then formula_26 is the set of all expensive cars.
formula_64<br>
It is also called the relative complement of formula_26 in formula_27 .<br>
"Example:" When formula_26 is the set of all cheap cars, and formula_27 is the set of all American cars, then formula_64 is the set of all expensive American cars.
If you exchange the sets in the set difference, the result is different:<br>In the "example" with the cars, the difference formula_70 is the set of all cheap cars, that are not made in America.
Most mathematicians use uppercase "ITALIC" (usually Roman) letters to write about sets (such as formula_26, formula_27, formula_73). The things that are seen as elements of sets are usually written with lowercase Roman letters.
One way of showing a set is by a list of its members, separated by commas, included in braces.
"For example",
Another way, called the set-builder notation, is by a statement of what is true of the members of the set, like this:
In spoken English, this reads: "the set of all "x" such that "x" is a natural number and "x" is less than four". The symbol [ipe "|" means "such that" or "so that".
The empty set is written in a special way: formula_75, formula_76 or formula_77.
When object "a" is the member of set formula_26 it is written as:
In spoken English, this reads: ""a" is a member of formula_26".
To illustrate operations on sets mathematicians use Venn diagrams. Venn diagrams use circles to show individual sets. The universe is depicted with a rectangle. Results of operations are shown as colored areas. In the illustration of the intersection operation the left circle shows set formula_26 and the right circle shows set formula_27.
Some sets are very important to mathematics. They are used very often. One of these is the "empty set".
Many of these special sets are written using blackboard bold typeface, and these include:
Each of these sets of numbers has an infinite number of elements, and formula_98.
The mathematician Bertrand Russell found that there are problems with the informal definition of sets. He stated this in a paradox called "Russell's paradox". An easier to understand version, closer to real life, is called the "Barber paradox".
There is a small town somewhere. In that town, there is a barber. All the men in the town do not like beards, so they either shave themselves, or they go to the barber shop to be shaved by the barber.
We can therefore make a statement about the barber himself: "The barber shaves all men that do not shave themselves. He only shaves those men (since the others shave themselves and do not need a barber to give them a shave)."
This of course raises the question: What does the barber do each morning to look clean-shaven? This is the paradox.
If the barber shaves himself, he cannot be a barber, since a barber does not shave himself.
If he does not shave himself, he falls in the category of those who do not shave themselves, and so, cannot be a barber.
The following books explore sets in more detail:
The Maldives are a group of islands in the Indian Ocean. They are southwest of India. They are made up of the Maldive and Suadive archipelagos, with over 1,200 islands. These islands are a part of 26 atolls. And they are divided to 20 administrative atolls. People live on about 200 of the islands. It has more than 1126 coral reefs. It is the flattest country in the world. The highest point in the Maldives is only high above the level of the sea.
Malé is the capital, with a population of 150,000 people. It’s official religion is Islam mandated by law.
The Maldives has a tropical monsoon climate (Am) under the Köppen climate classification, which is affected by the large landmass of South Asia to the north. Because the Maldives has the lowest elevation of any country in the world, the temperature is constantly hot and often humid. The presence of this landmass causes differential heating of land and water. These factors set off a rush of moisture-rich air from the Indian Ocean over South Asia, resulting in the southwest monsoon. Two seasons dominate Maldives' weather: the dry season associated with the winter northeastern monsoon and the rainy season which brings strong winds and storms.
The shift from the dry northeast monsoon to the moist southwest monsoon occurs during April and May. During this period, the southwest winds contribute to the formation of the southwest monsoon, which reaches Maldives in the beginning of June and lasts until the end of August. However, the weather patterns of Maldives do not always conform to the monsoon patterns of South Asia. The annual rainfall averages in the north and in the south.
The monsoonal influence is greater in the north of the Maldives than in the south, more influenced by the equatorial currents.
The highest temperature ever recorded in Malé was 39.6 °C (103.3 °F) on 1 January 2000 and the lowest was 20.4 °C (68.7 °F) on 20 March 1989 and 21 January 2017.
Prime Minister of Australia
The Prime Minister of Australia is the head of the executive branch of the government of Australia. The prime leads the cabinet of Australia and is the highest ranking Minister of the Crown in the federal parliament. The prime minister is appointed by the monarch, who is advised by the governor general.
The current Prime Minister of Australia, since August 2018, is Scott Morrison.
As of , there are six living former Australian Prime Ministers.
1190s
In science, structure is a fundamental concept. It means the arrangement of parts. Examples are the organs in a body, or crystals in a substance like granite.
A structure may refer to buildings, machines and similar things made by people as well as to a rock or a mineral, atoms or sentences. It always reflects the hierarchy of its parts.
Hardcore punk
Hardcore punk (or hardcore) is a style of punk music that usually sounds louder, faster and more angry than earlier punk rock music from the 1970s. Hardcore began in the late 1970s. Some of the important bands of the genre are Bad Brains, Black Flag, D.O.A, Minor Threat and Cro-Mags. Hardcore songs often have more political lyrics (words) than songs from 1970s punk.
No one is quite sure where the term "hardcore punk" came from. The band D.O.A. may have helped to make the term more popular with the title of their album "Hardcore '81" in 1981.
The first hardcore punk song was "Out of Vogue" by the Santa Ana band The Middle Class. This band started to play a loud type of punk with shouted words.
People in hardcore punk bands and people who listened to hardcore punk wore clothes that were of two types. Some people wore clothes and hairstyles that were strange and a bit scary, like Mohawk hairstyles and leather jackets with metal studs. Other people wore regular clothes like black pants and T-shirts.
At hardcore punk concerts, people did a type of dancing called moshing. This type of dance involves people pushing each other and jumping off the stage.
In the early 1980s, hardcore bands from England began to get noticed, such as Discharge, The Exploited, and The Varukers. Discharge played with a sound that included heavy metal guitar styles.
Hardcore led to the creation of the straight edge movement. People who are straight edge do not use alcohol, tobacco, and other drugs. Straight edge led to two new groups, called hardline and youth crew. Hardcore musicians also created independent record labels in the 1980s.
Many bands had a "Do it Yourself" (DIY) attitude. Bands would record their own albums using simple four-track recorders, and the sell the records themselves at shows or through the mail. Hardcore influenced the creation of new styles of music, including alternative rock, grunge, alternative metal, metalcore, thrash metal and post-hardcore.
Many early hardcore punk bands had songs about liberal politics such as anarchism and socialism. They spoke against political leaders, such as American president Ronald Reagan and British prime minister Margaret Thatcher. While George W. Bush was President of the United States, a number of hardcore bands criticized him.
A small number of hardcore musicians have expressed right wing views, including the band Antiseen, whose guitarist Joe Young ran for office as a Libertarian in North Carolina. Former Misfits singer Michale Graves supported George W. Bush.
Hardcore band singers often sing by shouting the words of the song. Hardcore songs often sound like the singer is angry.
Hardcore bands use electric guitars, electric bass guitar, drums, and a singer. The electric guitars in hardcore bands are distorted, noisy, and fuzzy because the guitarists are using electronic boxes called distortion pedals. Some hardcore guitarists have changed the tuning of their guitar strings so they play notes that are lower than those a normal guitar can play.
The drums in hardcore music play loud and fast, often characterized by use of the double-bass pedal, particularly notable in the breakdowns.