Some migratory animals know their locations by the intensity of the field. They know the time because of circadian rhythms the field produces. Migratory animals are born with a magnetic map in their head that allows them to migrate great distances safely. Their ability to sense the magnetic field is because of magnetic particles. Other animals have a chemical compass based on a radical pair mechanism.
Atmosphere (unit)
The standard atmosphere (symbol: atm) is a unit of pressure that refers to the air pressure at sea level. It is defined as equal to 101325 Pa. Since 1982, it has been replaced by the bar, which is equal to exactly 10 Pa. The usage of either unit is discouraged.
The 40-Year-Old Virgin
The 40-Year-Old Virgin is a 2005 American romantic comedy movie. It was written, directed and co-produced by Judd Apatow.
Steve Carell plays the eponymous virgin who lives alone and is involuntarily celibate. He works in the stockroom of an electronics store and is socially inept. Carell also wrote the script. The movie got mostly good reviews from critics.
Wilga Rivers
Wilga Rivers (13 April 1919 - 23 June 2007) was an Australian academic and writer. She was a Professor Emerita of Romance Languages and Literatures at Harvard University. She studied and wrote about teaching languages. Rivers was an early supporter of the change from the audiolingual method to more communicative and interactive language teaching. Rivers was born in Melbourne, Australia and died in Watertown, Massachusetts.
List of women linguists
Mount Curdie
Mount Curdie is a mountain in the Northern Territory of Australia. It is 958 metres high. It is the highest peak in the Curdie Range, which is a section of the Petermann Ranges. The mountain is located on the southern bank of the Hull River, about southeast of Kaḻṯukatjara. The Great Central Road passes nearby to the north. At the base of the mountain is the valley through which the Hull River flows. The valley contains a place known to local Aboriginal people as Tjunti, where Lasseter's Cave is located.
Ernest Giles named Mount Curdie on 13 March 1874, during his first journey through the Petermann Ranges. He named it after Dr Daniel Curdie, a subscriber to Giles' fund for his expedition. After climbing to the top of Mount Curdie, Giles also gave names to other peaks in the Petermann Ranges, including Mounts Fagan and Miller further to the east.
Sarah Caldwell
Sarah Caldwell (born March 6, 1924 - died March 23, 2006) was an American opera conductor and a stage director. She sang, performed and was director of opera music. Sarah was born in Missouri, though was raised in Arkansas. She later died due to heart failure in Portland, Maine at 82.
Guangzhou Evergrande
Guangzhou Evergrande is a football club which plays in China. This club has been very successful ever since Evergrande RE took over the club after a match-fixing scandal in 2009, with money flowing in.
Amy Madigan
Amy Madigan (September 11, 1950) is an American television and movie actress. She has acted in many notable movies and TV series. These include the 1989 movie "Field of Dreams" and "Uncle Buck" and the series "Grey's Anatomy", "Carnivàle" and "Fringe". She was born in Chicago.
Prolog
Prolog is a programming language that uses first order logic. It is the most used logic programming language. It is a declarative programming language. Alain Cormerauer, a French computer scientist created Prolog in the early 1970s.
Prolog is rather different from other programming languages. It uses facts and rules. Given the facts, the rules can be used to figure out new facts.
Prolog uses a paradigm called negation as failure, which means that formula_1 is assumed if formula_2 cannot be derived. This is different from true logical negation. While negation is failure has its benefits, it often confuses people starting to learn Prolog, as they expect true logical negation.
Prolog also uses Horn clauses. It is a turing-complete programming language.
Graupel
Graupel is a form of precipitation, which is very similar to snow and hail. However, hail falls almost always during severe thunderstorms. Snow falls during blizzards or snowstorms. Graupel often falls in wintry mixes, and usually falls apart during contact.
Horn clause
A Horn clause is a logic disjunction of literals, where at most one of the literals is positive, and all the others are negative. It is named after Alfred Horn who described them in an article in 1951.
A Horn clause with exactly one positive literal is a definite clause; a definite clause with no negative literals is sometimes called a “fact”; and a Horn clause without a positive literal is sometimes called a goal clause. These three kinds of Horn clauses are illustrated in the following propositional example:
In the non-propositional case, all variables in a clause are implicitly universally quantified with scope the entire clause. Thus, for example:
stands for:
which is logically equivalent to:
Horn clauses play a basic role in constructive logic and computational logic. They are important in automated theorem proving by first-order resolution, because the resolvent of two Horn clauses is itself a Horn clause, and the resolvent of a goal clause and a definite clause is a goal clause. These properties of Horn clauses can lead to greater efficiencies in proving a theorem (represented as the negation of a goal clause).
Horn clauses are also the basis of logic programming, where it is common to write definite clauses in the form of an implication:
In fact, the resolution of a goal clause with a definite clause to produce a new goal clause is the basis of the SLD resolution inference rule, used to implement logic programming and the programming language Prolog.
In logic programming a definite clause behaves as a goal-reduction procedure. For example, the Horn clause written above behaves as the procedure:
To emphasize this backwards use of the clause, it is often written in the backward form:
In Prolog this is written as:
The Prolog notation is actually ambiguous, and the term “goal clause” is sometimes also used ambiguously. The variables in a goal clause can be read as universally or existentially quantified, and deriving “false” can be interpreted either as deriving a contradiction or as deriving a successful solution of the problem to be solved.
Van Emden and Kowalski (1976) investigated the model theoretic properties of Horn clauses in the context of logic programming, showing that every set of definite clauses D has a unique minimal model M. An atomic formula A is logically implied by D if and only if A is true in M. It follows that a problem P represented by an existentially quantified conjunction of positive literals is logically implied by D if and only if P is true in M. The minimal model semantics of Horn clauses is the basis for the stable model semantics of logic programs.
Propositional Horn clauses are also of interest in computational complexity, where the problem of finding truth value assignments to make a conjunction of propositional Horn clauses true is a P-complete problem (in fact solvable in linear time), sometimes called HORNSAT. (The unrestricted Boolean satisfiability problem is an NP-complete problem however.) Satisfiability of first-order Horn clauses is undecidable.
Venn diagram
A Venn diagram is a diagram that shows the logical relation between sets. They were popularised by John Venn in the 1880s, and are now widely used. They are used to teach elementary set theory, and to illustrate simple set relationships in probability, logic, statistics, linguistics and computer science. A Venn diagram uses closed curves drawn on a plane to represent sets. Very often, these curves are circles or ellipses.