[{"@context":"http:\/\/schema.org\/","@type":"BlogPosting","@id":"https:\/\/wiki.edu.vn\/en\/wiki24\/quadratic-set-wikipedia\/#BlogPosting","mainEntityOfPage":"https:\/\/wiki.edu.vn\/en\/wiki24\/quadratic-set-wikipedia\/","headline":"Quadratic set – Wikipedia","name":"Quadratic set – Wikipedia","description":"From Wikipedia, the free encyclopedia In mathematics, a quadratic set is a set of points in a projective space that","datePublished":"2019-07-17","dateModified":"2019-07-17","author":{"@type":"Person","@id":"https:\/\/wiki.edu.vn\/en\/wiki24\/author\/lordneo\/#Person","name":"lordneo","url":"https:\/\/wiki.edu.vn\/en\/wiki24\/author\/lordneo\/","image":{"@type":"ImageObject","@id":"https:\/\/secure.gravatar.com\/avatar\/c9645c498c9701c88b89b8537773dd7c?s=96&d=mm&r=g","url":"https:\/\/secure.gravatar.com\/avatar\/c9645c498c9701c88b89b8537773dd7c?s=96&d=mm&r=g","height":96,"width":96}},"publisher":{"@type":"Organization","name":"Enzyklop\u00e4die","logo":{"@type":"ImageObject","@id":"https:\/\/wiki.edu.vn\/wiki4\/wp-content\/uploads\/2023\/08\/download.jpg","url":"https:\/\/wiki.edu.vn\/wiki4\/wp-content\/uploads\/2023\/08\/download.jpg","width":600,"height":60}},"image":{"@type":"ImageObject","@id":"https:\/\/wikimedia.org\/api\/rest_v1\/media\/math\/render\/svg\/be986ea6fba5658cd3c5cd23ac95266d9ec41709","url":"https:\/\/wikimedia.org\/api\/rest_v1\/media\/math\/render\/svg\/be986ea6fba5658cd3c5cd23ac95266d9ec41709","height":"","width":""},"url":"https:\/\/wiki.edu.vn\/en\/wiki24\/quadratic-set-wikipedia\/","about":["Wiki"],"wordCount":7716,"articleBody":"From Wikipedia, the free encyclopediaIn mathematics, a quadratic set is a set of points in a projective space that bears the same essential incidence properties as a quadric (conic section in a projective plane, sphere or cone or hyperboloid in a projective space).Table of ContentsDefinition of a quadratic set[edit]Definition of an oval and an ovoid[edit]References[edit]External links[edit]Definition of a quadratic set[edit]Let P=(P,G,\u2208){displaystyle {mathfrak {P}}=({mathcal {P}},{mathcal {G}},in )} be a projective space. A quadratic set is a non-empty subset Q{displaystyle {mathcal {Q}}} of P{displaystyle {mathcal {P}}} for which the following two conditions hold:(QS1) Every line g{displaystyle g} of G{displaystyle {mathcal {G}}} intersects Q{displaystyle {mathcal {Q}}} in at most two points or is contained in Q{displaystyle {mathcal {Q}}}.(g{displaystyle g} is called exterior to Q{displaystyle {mathcal {Q}}} if |g\u2229Q|=0{displaystyle |gcap {mathcal {Q}}|=0}, tangent to Q{displaystyle {mathcal {Q}}} if either |g\u2229Q|=1{displaystyle |gcap {mathcal {Q}}|=1} or g\u2229Q=g{displaystyle gcap {mathcal {Q}}=g}, and secant to Q{displaystyle {mathcal {Q}}} if |g\u2229Q|=2{displaystyle |gcap {mathcal {Q}}|=2}.)(QS2) For any point P\u2208Q{displaystyle Pin {mathcal {Q}}} the union QP{displaystyle {mathcal {Q}}_{P}} of all tangent lines through P{displaystyle P} is a hyperplane or the entire space P{displaystyle {mathcal {P}}}.A quadratic set Q{displaystyle {mathcal {Q}}} is called non-degenerate if for every point P\u2208Q{displaystyle Pin {mathcal {Q}}}, the set QP{displaystyle {mathcal {Q}}_{P}} is a hyperplane.A Pappian projective space is a projective space in which Pappus’s hexagon theorem holds.The following result, due to Francis Buekenhout, is an astonishing statement for finite projective spaces.Theorem: Let be Pn{displaystyle {mathfrak {P}}_{n}} a finite projective space of dimension n\u22653{displaystyle ngeq 3} and Q{displaystyle {mathcal {Q}}} a non-degenerate quadratic set that contains lines. Then: Pn{displaystyle {mathfrak {P}}_{n}} is Pappian and Q{displaystyle {mathcal {Q}}} is a quadric with index \u22652{displaystyle geq 2}.Definition of an oval and an ovoid[edit]Ovals and ovoids are special quadratic sets:Let P{displaystyle {mathfrak {P}}} be a projective space of dimension \u22652{displaystyle geq 2}. A non-degenerate quadratic set O{displaystyle {mathcal {O}}} that does not contain lines is called ovoid (or oval in plane case).The following equivalent definition of an oval\/ovoid are more common:Definition: (oval)A non-empty point set o{displaystyle {mathfrak {o}}} of a projective plane is calledoval if the following properties are fulfilled:(o1) Any line meets o{displaystyle {mathfrak {o}}} in at most two points.(o2) For any point P{displaystyle P} in o{displaystyle {mathfrak {o}}} there is one and only one line g{displaystyle g} such that  g\u2229o={P}{displaystyle gcap {mathfrak {o}}={P}}.A line g{displaystyle g}  is a exterior or tangent or secant line of theoval if |g\u2229o|=0{displaystyle |gcap {mathfrak {o}}|=0} or |g\u2229o|=1{displaystyle |gcap {mathfrak {o}}|=1} or |g\u2229o|=2{displaystyle |gcap {mathfrak {o}}|=2} respectively.For finite planes the following theorem provides a more simple definition.Theorem: (oval in finite plane) Let be P{displaystyle {mathfrak {P}}} a projective plane of order n{displaystyle n}.A set o{displaystyle {mathfrak {o}}} of points is an oval if |o|=n+1{displaystyle |{mathfrak {o}}|=n+1} and if no three pointsof o{displaystyle {mathfrak {o}}} are collinear.According to this theorem of Beniamino Segre, for Pappian projective planes of odd order the ovals are just conics:Theorem:Let be P{displaystyle {mathfrak {P}}} a Pappian projective plane of odd order.Any oval in P{displaystyle {mathfrak {P}}}  is an oval conic (non-degenerate quadric).Definition: (ovoid)A non-empty point set O{displaystyle {mathcal {O}}} of a projective space is called ovoid if the following properties are fulfilled:(O1) Any line meets O{displaystyle {mathcal {O}}} in at most two points.(g{displaystyle g} is called exterior, tangent and  secant line if |g\u2229O|=0,\u00a0|g\u2229O|=1{displaystyle |gcap {mathcal {O}}|=0, |gcap {mathcal {O}}|=1} and |g\u2229O|=2{displaystyle |gcap {mathcal {O}}|=2} respectively.)(O2) For any point P\u2208O{displaystyle Pin {mathcal {O}}} the union OP{displaystyle {mathcal {O}}_{P}} of all tangent lines through P{displaystyle P} is a hyperplane (tangent plane at P{displaystyle P}).Example:a) Any sphere (quadric of index 1) is an ovoid.b) In case of real projective spaces one can construct ovoids by combining halves of suitable ellipsoids such that they are no quadrics.For finite projective spaces of dimension n{displaystyle n} over a field K{displaystyle K} we have:Theorem:a) In case of |K|,\u00a0char\u2061K\u22602{displaystyle |K|  "},{"@context":"http:\/\/schema.org\/","@type":"BreadcrumbList","itemListElement":[{"@type":"ListItem","position":1,"item":{"@id":"https:\/\/wiki.edu.vn\/en\/wiki24\/#breadcrumbitem","name":"Enzyklop\u00e4die"}},{"@type":"ListItem","position":2,"item":{"@id":"https:\/\/wiki.edu.vn\/en\/wiki24\/quadratic-set-wikipedia\/#breadcrumbitem","name":"Quadratic set – Wikipedia"}}]}]