De Morgan's laws
In propositional logic and Boolean algebra, De Morgan's laws are a pair of transformation rules that are both valid rules of inference. They are named after Augustus De Morgan, a 19th-century British mathematician. The rules allow the expression of conjunctions and disjunctions purely in terms of each other via negation. The rules can be expressed in English as: or or where "A or B" is an "inclusive or" meaning "at least" one of A or B rather than an "exclusive or" that means "exactly" one of A or B. In set theory and Boolean  algebra, these are written formally as where In formal language, the rules are written as and where Applications of the rules include simplification of logical expressions in computer programs and digital circuit designs. De Morgan's laws are an example of a more general concept of mathematical duality. Formal notation. The "negation of conjunction" rule may be written in sequent notation: and The "negation of disjunction" rule may be written as: and In rule form: "negation of conjunction" and "negation of disjunction" and expressed as a truth-functional tautology or theorem of propositional logic: where formula_23 and formula_24 are propositions  expressed in some formal system. Substitution form. De Morgan's laws are normally shown in the compact form above, with the negation of the output on the left and negation of the inputs on the right. A clearer form for substitution can be stated as: This emphasizes the need to invert both the inputs and the output, as well as change the operator when doing a substitution. The laws have an important gap to the (formula_26) double negation law. formula_27, to become a formal logic system: formula_28 sequence reports symbols that are defined well formed at first order. The same  system has those conjunctions: formula_29. Obviously, formula_30 is valid knowledge, then there is at least one formula_31 conjunction, which -formula_32 number —in the truth table, basic proposition of formula_31— is equal to atomic existence context of formula_31, of course according to the formula_35 knowledge. We regarded the equivalence theory, the logic does. At this point, De Morgan Laws show effect that is upward or downward, the atomic context of formula_31. Set theory and Boolean algebra. In set theory and Boolean algebra, it is often stated as "union and intersection interchange under complementation", which can be formally expressed as: where: Infinite  unions and intersections. The generalized form is where is some, possibly uncountable, indexing set. In set notation, De Morgan's laws can be remembered using the mnemonic "break the line, change the sign". Engineering. In electrical and computer engineering, De Morgan's laws are commonly written as: and where: Text searching. De Morgan's laws commonly apply to text searching using Boolean operators AND, OR, and NOT. Consider a set of documents containing the words "cars" and "trucks". De Morgan's laws hold that these two searches will return the same set of documents: The corpus of documents containing "cars  or trucks" can be represented by four documents: To evaluate Search A, clearly the search "(cars OR trucks)" will hit on Documents 1, 2, and 3. So the negation of that search (which is Search A) will hit everything else, which is Document 4. Evaluating Search B, the search "(NOT cars)" will hit on documents that do not contain "cars", which is Documents 2 and 4. Similarly the search "(NOT trucks)" will hit on Documents 1 and 4. Applying the AND operator to these two searches (which is Search B) will hit on the documents that are common to these  two searches, which is Document 4. A similar evaluation can be applied to show that the following two searches will return the same set of documents (Documents 1, 2, 4): History. The laws are named after Augustus De Morgan (1806–1871), who introduced a formal version of the laws to classical propositional logic. De Morgan's formulation was influenced by algebraization of logic undertaken by George Boole, which later cemented De Morgan's claim to the find. Nevertheless, a similar observation was made by Aristotle, and was known to Greek and Medieval logicians. For example, in the 14th century, William of  Ockham wrote down the words that would result by reading the laws out. Jean Buridan, in his "Summulae de Dialectica", also describes rules of conversion that follow the lines of De Morgan's laws. Still, De Morgan is given credit for stating the laws in the terms of modern formal logic, and incorporating them into the language of logic. De Morgan's laws can be proved easily, and may even seem trivial. Nonetheless, these laws are helpful in making valid inferences in proofs and deductive arguments. Informal proof. De Morgan's theorem may be applied to the negation of a  disjunction or the negation of a conjunction in all or part of a formula. Negation of a disjunction. In the case of its application to a disjunction, consider the following claim: "it is false that either of A or B is true", which is written as: In that it has been established that "neither" A nor B is true, then it must follow that both A is not true and B is not true, which may be written directly as: If either A or B "were" true, then the disjunction of A and B would be true, making its negation  false. Presented in English, this follows the logic that "since two things are both false, it is also false that either of them is true". Working in the opposite direction, the second expression asserts that A is false and B is false (or equivalently that "not A" and "not B" are true). Knowing this, a disjunction of A and B must be false also. The negation of said disjunction must thus be true, and the result is identical to the first claim. Negation of a conjunction. The application of De Morgan's theorem to conjunction is very similar to its  application to a disjunction both in form and rationale. Consider the following claim: "it is false that A and B are both true", which is written as: In order for this claim to be true, either or both of A or B must be false, for if they both were true, then the conjunction of A and B would be true, making its negation false. Thus, one (at least) or more of A and B must be false (or equivalently, one or more of "not A" and "not B" must be true). This may be written directly as, Presented in  English, this follows the logic that "since it is false that two things are both true, at least one of them must be false". Working in the opposite direction again, the second expression asserts that at least one of "not A" and "not B" must be true, or equivalently that at least one of A and B must be false. Since at least one of them must be false, then their conjunction would likewise be false. Negating said conjunction thus results in a true expression, and this expression is identical to the first claim. Formal proof. Here we use formula_51to  denote the complement of A. The proof that formula_52 is completed in 2 steps by proving both formula_53 and formula_54. Part 1. Let formula_55. Then, formula_56. Because formula_57, it must be the case that formula_58 or formula_59. If formula_58, then formula_61, so formula_62. Similarly, if formula_59, then formula_64, so formula_65. Thus, formula_66; that is, formula_53. Part 2. To prove the reverse direction, let formula_62, and for contradiction assume formula_69. Under that assumption, it must be the case that formula_70, so it follows that formula_71 and formula_72, and thus formula_73 and formula_74. However, that means formula_75, in contradiction to the hypothesis  that formula_62, therefore, the assumption formula_69 must not be the case, meaning that formula_78. Hence, formula_79, that is, formula_54. Conclusion. If formula_54 "and" formula_82, then formula_52; this concludes the proof of De Morgan's law. The other De Morgan's law, formula_84, is proven similarly. Generalising De Morgan duality. In extensions of classical propositional logic, the duality still holds (that is, to any logical operator one can always find its dual), since in the presence of the identities governing negation, one may always introduce an operator that is the De Morgan dual of another. This leads to an important property  of logics based on classical logic, namely the existence of negation normal forms: any formula is equivalent to another formula where negations only occur applied to the non-logical atoms of the formula. The existence of negation normal forms drives many applications, for example in digital circuit design, where it is used to manipulate the types of logic gates, and in formal logic, where it is needed to find the conjunctive normal form and disjunctive normal form of a formula. Computer programmers use them to simplify or properly negate complicated logical conditions. They are also often useful in computations in  elementary probability theory. Let one define the dual of any propositional operator P("p", "q", ...) depending on elementary propositions "p", "q", ... to be the operator formula_85 defined by Extension to predicate and modal logic. This duality can be generalised to quantifiers, so for example the universal quantifier and existential quantifier are duals: To relate these quantifier dualities to the De Morgan laws, set up a model with some small number of elements in its domain "D", such as Then and But, using De Morgan's laws, and verifying the quantifier dualities in the model. Then, the quantifier dualities can be extended  further to modal logic, relating the box ("necessarily") and diamond ("possibly") operators: In its application to the alethic modalities of possibility and necessity, Aristotle observed this case, and in the case of normal modal logic, the relationship of these modal operators to the quantification can be understood by setting up models using Kripke semantics. In intuitionistic logic. Three out of the four implications of de Morgan's laws hold in intuitionistic logic. Specifically, we have and while the converse of the last implication does not hold in pure intuitionistic logic and would be equivalent to the law of the weak  excluded middle which can be used as a foundation for an intermediate logic. In popular culture. The 2nd most-famous quote from Sherlock Holmes stories, used several times, is a restatement of de Morgan's law: How often have I said to you that when you have eliminated the impossible, whatever remains, however improbable, must be the truth?