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A plane duality is a map from a projective plane C = (P, L, I) to its dual plane C ∗ = (L, P, I ∗) (see § Principle of duality above) which preserves incidence. That is, a plane duality σ will map points to lines and lines to points (P σ = L and L σ = P) in such a way that if a point Q is on a line m (denoted by Q I m) then Q I m ⇔ m ...
One definition of a red-eye flight is one that is too short to allow a full night's sleep. An example would be those flights from Los Angeles to New York City —about five hours' flying time—that depart between 2100 and 0100 Pacific time , and arrive between 0500 and 0900 Eastern time .
The same term can also be used more informally to refer to something "standard" or "classic". For example, one might say that Euclid's proof is the "canonical proof" of the infinitude of primes. There are two canonical proofs that are always used to show non-mathematicians what a mathematical proof is like:
In mathematics, a plane is a two-dimensional space or flat surface that extends indefinitely. A plane is the two-dimensional analogue of a point (zero dimensions), a line (one dimension) and three-dimensional space. When working exclusively in two-dimensional Euclidean space, the definite article is used, so the Euclidean plane refers to the ...
For example: "All humans are mortal, and Socrates is a human. ∴ Socrates is mortal." ∵ Abbreviation of "because" or "since". Placed between two assertions, it means that the first one is implied by the second one. For example: "11 is prime ∵ it has no positive integer factors other than itself and one." ∋ 1. Abbreviation of "such that".
A line arrangement in the projective plane differs from its Euclidean counterpart in that the two Euclidean rays at either end of a line are replaced by a single edge in the projective plane that connects the leftmost and rightmost vertices on that line, and in that pairs of unbounded Euclidean cells are replaced in the projective plane by ...
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The archetypical example is the real projective plane, also known as the extended Euclidean plane. [1] This example, in slightly different guises, is important in algebraic geometry , topology and projective geometry where it may be denoted variously by PG(2, R ) , RP 2 , or P 2 ( R ), among other notations.