Anvelopă convexă: Diferență între versiuni
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definiții, biblio |
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La fel ca pentru mulțimi de puncte, anvelopa convexă a fost studiată pentru [[poligon simplu |poligoane simple]], [[mișcare browniană]], curbe în spațiu și {{ill-wd|Q1347059||epigrafele funcțiilor}}. Anvelopa convexă are numeroase aplicații în matematică, statistică, optimizare combinatorială, economie, modelare geometrică și etologie. Structurile conexe includ {{ill-wd|Q7104528||anvelopa convexă ortogonală}}, {{ill-wd|Q28136697||straturile convexe}}, [[triangulația Delaunay]] și {{ill-wd|Q757267||placarea Voronoi}}. |
La fel ca pentru mulțimi de puncte, anvelopa convexă a fost studiată pentru [[poligon simplu |poligoane simple]], [[mișcare browniană]], curbe în spațiu și {{ill-wd|Q1347059||epigrafele funcțiilor}}. Anvelopa convexă are numeroase aplicații în matematică, statistică, optimizare combinatorială, economie, modelare geometrică și etologie. Structurile conexe includ {{ill-wd|Q7104528||anvelopa convexă ortogonală}}, {{ill-wd|Q28136697||straturile convexe}}, [[triangulația Delaunay]] și {{ill-wd|Q757267||placarea Voronoi}}. |
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== Definiții == |
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[[Fișier:ConvexHull.svg|thumb|Anvelopa convexă a unei mulțimi mărginite din plan: analogia firului de cauciuc]] |
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O mulțime de puncte din spațiul euclidian este definită drept convexă dacă conține toate segmentele care unesc oricare pereche de puncte ale sale. Anvelopa convexă a unei mulțimi <math>X</math> date poate fi definită prin:{{sfnp|Rockafellar|1970|page=12}} |
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# Mulțimea convexă minima (unică) conținând <math>X</math> |
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# Intersecția tuturor mulțimilor convexe conținând <math>X</math> |
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# Mulțimea tuturor combinațiilor convexe de puncte din <math>X</math> |
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# Reuniunea tuturor [[simplex]]urilor cu vârfuri în <math>X</math> |
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Pentru o mulțime cu frontieră din planul euclidian, cu elemente necoliniare, frontiera anvelopei convexe este linia închisă cu [[perimetru]]l minim conținând pe <math>X</math>. Se poate imagina întinderea unui fir de cauciuc în jurul întregii mulțimi <math>S</math>, iar apoi, eliberându-l, el se strânge; când se oprește, el închide anvelopa convexă a <math>S</math>.{{sfnp|de Berg|van Kreveld|Overmars|Schwarzkopf|2008|page=3}} Această formulare nu se poate generaliza imediat pentru dimensiuni superioare: pentru o mulțime finită de puncte din spațiul tridimensional, o vecinătate a {{ill-wd| Q831672||arborelui de acoperire}} al punctelor le include cu o suprafață arbitrar de mică, mai mică decât suprafața anvelopei convexe.<ref>{{harvtxt|Williams|Rossignac|2005}}. Vezi și Douglas Zare, [https://mathoverflow.net/a/166317/440 answer to "the perimeter of a non-convex set"], [[MathOverflow]], May 16, 2014.</ref> Cu toate acestea, în dimensiuni superioare, variante ale {{ill-wd|Q3406250||problemei obstacolelor}} de a găsi o suprafață cu energie minimă pe deasupra unei forme date poate avea drept soluție anvelopa convexă.{{sfnp|Oberman|2007}} |
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Pentru obiectele tridimensionale, prima definiție afirmă că anvelopa convexă este cel mai mic posibil {{ill-wd| Q1434060}} convex al obiectelor. |
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Definiția pe baza intersecției mulțimilor convexe poate fi extinsă la [[geometrie neeuclidiană |geometriile neeuclidiene]], iar definiția pe baza combinațiilor convexe poate fi extinsă la un [[spațiu vectorial]] real sau la un [[spațiu afin]] oarecare; anvelopa convexă poate fi generalizată abstract în cadrul {{ill-wd|Q16155113||matroizilor orientați}}.{{sfnp|Knuth|1992}} |
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== Note == |
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{{listănote}} |
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{{refend}} |
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== Legături externe == |
== Legături externe == |
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[[Categorie:Operatori de închidere]] |
[[Categorie:Operatori de închidere]] |
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[[Categorie:Analiză convexă]] |
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[[Categorie:Geometrie algoritmică]] |
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<!-- [[Categorie:Geometrie convexă]] --> |
Versiunea de la 7 ianuarie 2021 20:11
Unul sau mai mulți editori lucrează în prezent la această pagină sau secțiune. Pentru a evita conflictele de editare și alte confuzii creatorul solicită ca, pentru o perioadă scurtă de timp, această pagină să nu fie editată inutil sau nominalizată pentru ștergere în această etapă incipientă de dezvoltare, chiar dacă există unele lacune de conținut. Dacă observați că nu au mai avut loc modificări de 10 zile puteți șterge această etichetă. |
![](http://upload.wikimedia.org/wikipedia/commons/thumb/8/8e/Extreme_points.svg/220px-Extreme_points.svg.png)
În geometrie, anvelopa convexă sau închiderea convexă a unei forme este cea mai mică mulțime convexă care o conține. Anvelopa convexă poate fi definită fie ca intersecția tuturor mulțimilor convexe care conțin o submulțime dată a spațiului euclidian, fie ca mulțimea tuturor combinațiilor convexe(d) ale punctelor din submulțimea dată. Pentru o porțiune dintr-un plan, anvelopa convexă poate fi vizualizată printr-un fir de cauciuc întins în jurul porțiunii.
Anvelopa convexă a unei mulțimi deschise este una deschisă, iar anvelopa convexă a unei mulțimi compacte este una compactă. Orice mulțime convexă compactă este anvelopa convexă a punctelor sale extreme(d). Operatorul „anvelopă convexă” este un exemplu de operator de închidere(d), iar orice antimatroid(d) poate fi reprezentat prin aplicarea acestui operator de închidere la o mulțime finită de puncte. Problema algoritmilor de trasare a anvelopei convexe a unei mulțimi finite de puncte din plan sau alte spații euclidiene cu dimensiuni inferioare, precum și problema duală a intersectării semispațiilor, sunt probleme fundamentale ale geometriei algoritmice(d). Pentru mulțimi situate în plan sau în spațiul tridimensional, acestea pot fi rezolvate cu resurse de calcul de complexitatea , iar pentru dimensiuni superioare, în cel mai rău caz cu resurse de calcul de complexitatea dată de teorema limitei superioare(d).
La fel ca pentru mulțimi de puncte, anvelopa convexă a fost studiată pentru poligoane simple, mișcare browniană, curbe în spațiu și epigrafele funcțiilor(d). Anvelopa convexă are numeroase aplicații în matematică, statistică, optimizare combinatorială, economie, modelare geometrică și etologie. Structurile conexe includ anvelopa convexă ortogonală(d), straturile convexe(d), triangulația Delaunay și placarea Voronoi(d).
Definiții
![](http://upload.wikimedia.org/wikipedia/commons/thumb/d/de/ConvexHull.svg/220px-ConvexHull.svg.png)
O mulțime de puncte din spațiul euclidian este definită drept convexă dacă conține toate segmentele care unesc oricare pereche de puncte ale sale. Anvelopa convexă a unei mulțimi date poate fi definită prin:[1]
- Mulțimea convexă minima (unică) conținând
- Intersecția tuturor mulțimilor convexe conținând
- Mulțimea tuturor combinațiilor convexe de puncte din
- Reuniunea tuturor simplexurilor cu vârfuri în
Pentru o mulțime cu frontieră din planul euclidian, cu elemente necoliniare, frontiera anvelopei convexe este linia închisă cu perimetrul minim conținând pe . Se poate imagina întinderea unui fir de cauciuc în jurul întregii mulțimi , iar apoi, eliberându-l, el se strânge; când se oprește, el închide anvelopa convexă a .[2] Această formulare nu se poate generaliza imediat pentru dimensiuni superioare: pentru o mulțime finită de puncte din spațiul tridimensional, o vecinătate a arborelui de acoperire(d) al punctelor le include cu o suprafață arbitrar de mică, mai mică decât suprafața anvelopei convexe.[3] Cu toate acestea, în dimensiuni superioare, variante ale problemei obstacolelor(d) de a găsi o suprafață cu energie minimă pe deasupra unei forme date poate avea drept soluție anvelopa convexă.[4]
Pentru obiectele tridimensionale, prima definiție afirmă că anvelopa convexă este cel mai mic posibil volum de delimitare(d) convex al obiectelor. Definiția pe baza intersecției mulțimilor convexe poate fi extinsă la geometriile neeuclidiene, iar definiția pe baza combinațiilor convexe poate fi extinsă la un spațiu vectorial real sau la un spațiu afin oarecare; anvelopa convexă poate fi generalizată abstract în cadrul matroizilor orientați(d).[5]
Note
- ^ Rockafellar (1970), p. 12.
- ^ de Berg et al. (2008), p. 3.
- ^ Williams & Rossignac (2005). Vezi și Douglas Zare, answer to "the perimeter of a non-convex set", MathOverflow, May 16, 2014.
- ^ Oberman (2007).
- ^ Knuth (1992).
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Legături externe
- en Convex hull (română Anvelopa convexă) la Encyclopedia of Mathematics, EMS Press, Springer, 2001
- en Convex Hull, (română Anvelopa convexă) la Mathworld