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  2. De Morgan's laws - Wikipedia

    en.wikipedia.org/wiki/De_Morgan's_laws

    De Morgan's laws represented with Venn diagrams.In each case, the resultant set is the set of all points in any shade of blue. In propositional logic and Boolean algebra, De Morgan's laws, [1] [2] [3] also known as De Morgan's theorem, [4] are a pair of transformation rules that are both valid rules of inference.

  3. Associative property - Wikipedia

    en.wikipedia.org/wiki/Associative_property

    For example, the order does not matter in the multiplication of real numbers, that is, a × b = b × a, so we say that the multiplication of real numbers is a commutative operation. However, operations such as function composition and matrix multiplication are associative, but not (generally) commutative.

  4. Inclusion–exclusion principle - Wikipedia

    en.wikipedia.org/wiki/Inclusion–exclusion...

    The board B is any subset of the squares of a rectangular board with n rows and m columns; we think of it as the squares in which one is allowed to put a rook. The coefficient, r k (B) of x k in the rook polynomial R B (x) is the number of ways k rooks, none of which attacks another, can be arranged in the squares of B.

  5. Signed measure - Wikipedia

    en.wikipedia.org/wiki/Signed_measure

    What follows are two results which will imply that an extended signed measure is the difference of two non-negative measures, and a finite signed measure is the difference of two finite non-negative measures. The Hahn decomposition theorem states that given a signed measure μ, there exist two measurable sets P and N such that: PN = X and P ...

  6. Proof of impossibility - Wikipedia

    en.wikipedia.org/wiki/Proof_of_impossibility

    One of the widely used types of impossibility proof is proof by contradiction.In this type of proof, it is shown that if a proposition, such as a solution to a particular class of equations, is assumed to hold, then via deduction two mutually contradictory things can be shown to hold, such as a number being both even and odd or both negative and positive.

  7. Proof by contradiction - Wikipedia

    en.wikipedia.org/wiki/Proof_by_contradiction

    A typical example is the proof of the proposition "there is no smallest positive rational number": assume there is a smallest positive rational number q and derive a contradiction by observing that ⁠ q / 2 ⁠ is even smaller than q and still positive.

  8. AOL Mail - AOL Help

    help.aol.com/products/aol-webmail

    Get answers to your AOL Mail, login, Desktop Gold, AOL app, password and subscription questions. Find the support options to contact customer care by email, chat, or phone number.

  9. Reductio ad absurdum - Wikipedia

    en.wikipedia.org/wiki/Reductio_ad_absurdum

    Reductio ad absurdum, painting by John Pettie exhibited at the Royal Academy in 1884. In logic, reductio ad absurdum (Latin for "reduction to absurdity"), also known as argumentum ad absurdum (Latin for "argument to absurdity") or apagogical arguments, is the form of argument that attempts to establish a claim by showing that the opposite scenario would lead to absurdity or contradiction.