If And Only If
In logic and related fields such as mathematics and philosophy, "if and only if" (shortened as "iff") is a biconditional logical connective between statements, where either both statements are true or both are false. The connective is biconditional (a statement of material equivalence), and can be likened to the standard material conditional ("only if", equal to "if ... then") combined with its reverse ("if"); hence the name. The result is that the truth of either one of the connected statements requires the truth of the other (i.e. either both statements are true, or both are false), though it is controversial whether the connective thus defined is properly rendered by the English "if and only if"—with its preexisting meaning. For example, ''P if and only if Q'' means that ''P'' is true whenever ''Q'' is true, and the only case in which ''P'' is true is if ''Q'' is also true, whereas in the case of ''P if Q'', there could be other scenarios where ''P'' is true and ''Q ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] 

Logic
Logic is the study of correct reasoning. It includes both formal and informal logic. Formal logic is the science of deductively valid inferences or of logical truths. It is a formal science investigating how conclusions follow from premises in a topicneutral way. When used as a countable noun, the term "a logic" refers to a logical formal system that articulates a proof system. Formal logic contrasts with informal logic, which is associated with informal fallacies, critical thinking, and argumentation theory. While there is no general agreement on how formal and informal logic are to be distinguished, one prominent approach associates their difference with whether the studied arguments are expressed in formal or informal languages. Logic plays a central role in multiple fields, such as philosophy, mathematics, computer science, and linguistics. Logic studies arguments, which consist of a set of premises together with a conclusion. Premises and conclusions are usua ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] 

Propositional Logic
Propositional calculus is a branch of logic. It is also called propositional logic, statement logic, sentential calculus, sentential logic, or sometimes zerothorder logic. It deals with propositions (which can be true or false) and relations between propositions, including the construction of arguments based on them. Compound propositions are formed by connecting propositions by logical connectives. Propositions that contain no logical connectives are called atomic propositions. Unlike firstorder logic, propositional logic does not deal with nonlogical objects, predicates about them, or quantifiers. However, all the machinery of propositional logic is included in firstorder logic and higherorder logics. In this sense, propositional logic is the foundation of firstorder logic and higherorder logic. Explanation Logical connectives are found in natural languages. In English for example, some examples are "and" ( conjunction), "or" (disjunction), "not" (negation) and "if" ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] 

Euler Diagram
An Euler diagram (, ) is a diagrammatic means of representing sets and their relationships. They are particularly useful for explaining complex hierarchies and overlapping definitions. They are similar to another set diagramming technique, Venn diagrams. Unlike Venn diagrams, which show all possible relations between different sets, the Euler diagram shows only relevant relationships. The first use of "Eulerian circles" is commonly attributed to Swiss mathematician Leonhard Euler (1707–1783). In the United States, both Venn and Euler diagrams were incorporated as part of instruction in set theory as part of the new math movement of the 1960s. Since then, they have also been adopted by other curriculum fields such as reading as well as organizations and businesses. Euler diagrams consist of simple closed shapes in a twodimensional plane that each depict a set or category. How or whether these shapes overlap demonstrates the relationships between the sets. Each curve divid ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] 

Consonant Gemination
In phonetics and phonology, gemination (), or consonant lengthening (from Latin 'doubling', itself from '' gemini'' 'twins'), is an articulation of a consonant for a longer period of time than that of a singleton consonant. It is distinct from stress. Gemination is represented in many writing systems by a doubled letter and is often perceived as a doubling of the consonant.William Ham, ''Phonetic and Phonological Aspects of Geminate Timing'', p. 118 Some phonological theories use "doubling" as a synonym for gemination, others describe two distinct phenomena. Consonant length is a distinctive feature in certain languages, such as Arabic, Berber, Danish, Estonian, Hindi, Hungarian, Italian, Japanese, Kannada, Punjabi, Polish and Turkish. Other languages, such as English, do not have wordinternal phonemic consonant geminates. Consonant gemination and vowel length are independent in languages like Arabic, Japanese, Finnish and Estonian; however, in languages like Italian, ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] 

Euphony
Phonaesthetics (also spelled phonesthetics in North America) is the study of beauty and pleasantness associated with the sounds of certain words or parts of words. The term was first used in this sense, perhaps by during the mid20th century and derives . Speech sounds have many aesthetic qualities, some of which are subjectively regarded as euphonious (pleasing) or cacophonous (displeasing). Phonaesthetics remains a budding and often subjective field of study, with no scientifically or otherwise formally established definition; today, it mostly exists as a marginal branch of psychology, phonetics, or poetics. More broadly, the British linguist David Crystal has regarded phonaesthetics as the study of "phonaesthesia" (i.e., sound symbolism and phonesthemes): that not just words but even certain sound combinations carry meaning. For example, he shows that English speakers tend to associate unpleasantness with the sound ''sl'' in such words as ''sleazy'', ''slime'', ''slug'', a ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] 

Paul Halmos
Paul Richard Halmos ( hu, Halmos Pál; March 3, 1916 – October 2, 2006) was a Hungarianborn American mathematician and statistician who made fundamental advances in the areas of mathematical logic, probability theory, statistics, operator theory, ergodic theory, and functional analysis (in particular, Hilbert spaces). He was also recognized as a great mathematical expositor. He has been described as one of The Martians. Early life and education Born in Hungary into a Jewish family, Halmos arrived in the U.S. at 13 years of age. He obtained his B.A. from the University of Illinois, majoring in mathematics, but fulfilling the requirements for both a math and philosophy degree. He took only three years to obtain the degree, and was only 19 when he graduated. He then began a Ph.D. in philosophy, still at the Champaign–Urbana campus; but, after failing his masters' oral exams, he shifted to mathematics, graduating in 1938. Joseph L. Doob supervised his dissertation, titled ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] 

John L
John Lasarus Williams (29 October 1924 – 15 June 2004), known as John L, was a Welsh nationalist activist. Williams was born in Llangoed on Anglesey, but lived most of his life in nearby Llanfairpwllgwyngyll. In his youth, he was a keen footballer, and he also worked as a teacher. His activism started when he campaigned against the refusal of Brewer Spinks, an employer in Blaenau Ffestiniog, to permit his staff to speak Welsh. This inspired him to become a founder of Undeb y Gymraeg Fyw, and through this organisation was the main organiser of ''Sioe Gymraeg y Borth'' (the Welsh show for Menai Bridge using the colloquial form of its Welsh name).Colli John L Williams , '' BBC Cymru'', 15 June 2004 Williams also join ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] 

Truthfunction
In logic, a truth function is a function that accepts truth values as input and produces a unique truth value as output. In other words: The input and output of a truth function are all truth values; a truth function will always output exactly one truth value; and inputting the same truth value(s) will always output the same truth value. The typical example is in propositional logic, wherein a compound statement is constructed using individual statements connected by logical connectives; if the truth value of the compound statement is entirely determined by the truth value(s) of the constituent statement(s), the compound statement is called a truth function, and any logical connectives used are said to be truth functional. Classical propositional logic is a truthfunctional logic, in that every statement has exactly one truth value which is either true or false, and every logical connective is truth functional (with a correspondent truth table), thus every compound statement is a ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] 

Disjunction
In logic, disjunction is a logical connective typically notated as \lor and read aloud as "or". For instance, the English language sentence "it is raining or it is snowing" can be represented in logic using the disjunctive formula R \lor S , assuming that R abbreviates "it is raining" and S abbreviates "it is snowing". In classical logic, disjunction is given a truth functional semantics according to which a formula \phi \lor \psi is true unless both \phi and \psi are false. Because this semantics allows a disjunctive formula to be true when both of its disjuncts are true, it is an ''inclusive'' interpretation of disjunction, in contrast with exclusive disjunction. Classical proof theoretical treatments are often given in terms of rules such as disjunction introduction and disjunction elimination. Disjunction has also been given numerous nonclassical treatments, motivated by problems including Aristotle's sea battle argument, Heisenberg's uncertainty principle, as wel ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] 

Proof Theory
Proof theory is a major branchAccording to Wang (1981), pp. 3–4, proof theory is one of four domains mathematical logic, together with model theory, axiomatic set theory, and recursion theory. Barwise (1978) consists of four corresponding parts, with part D being about "Proof Theory and Constructive Mathematics". of mathematical logic that represents proofs as formal mathematical objects, facilitating their analysis by mathematical techniques. Proofs are typically presented as inductivelydefined data structures such as lists, boxed lists, or trees, which are constructed according to the axioms and rules of inference of the logical system. Consequently, proof theory is syntactic in nature, in contrast to model theory, which is semantic in nature. Some of the major areas of proof theory include structural proof theory, ordinal analysis, provability logic, reverse mathematics, proof mining, automated theorem proving, and proof complexity. Much research also focuses ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] 

Logical System
A formal system is an abstract structure used for inferring theorems from axioms according to a set of rules. These rules, which are used for carrying out the inference of theorems from axioms, are the logical calculus of the formal system. A formal system is essentially an "axiomatic system". In 1921, David Hilbert proposed to use such a system as the foundation for the knowledge in mathematics. A formal system may represent a welldefined system of abstract thought. The term ''formalism'' is sometimes a rough synonym for ''formal system'', but it also refers to a given style of notation, for example, Paul Dirac's bra–ket notation. Background Each formal system is described by primitive symbols (which collectively form an alphabet) to finitely construct a formal language from a set of axioms through inferential rules of formation. The system thus consists of valid formulas built up through finite combinations of the primitive symbols—combinations that are formed fro ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] 

Exclusive Nor
Logical equality is a logical operator that corresponds to equality in Boolean algebra and to the logical biconditional in propositional calculus. It gives the functional value ''true'' if both functional arguments have the same logical value, and '' false'' if they are different. It is customary practice in various applications, if not always technically precise, to indicate the operation of logical equality on the logical operands ''x'' and ''y'' by any of the following forms: :\begin x &\leftrightarrow y & x &\Leftrightarrow y & \mathrm Exy \\ x &\mathrm y & x &= y \end Some logicians, however, draw a firm distinction between a ''functional form'', like those in the left column, which they interpret as an application of a function to a pair of arguments — and thus a mere indication that the value of the compound expression depends on the values of the component expressions — and an ''equational form'', like those in the right column, which they interpret as a ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] 