en>InverseHypercube: /* Digital dividers */

2013-10-30T19:46:44Z

Digital dividers

← Older revision		Revision as of 21:46, 30 October 2013
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	~~Andera~~ is ~~what you can contact her but she~~ by ~~no means truly favored that name~~. I'~~ve usually loved living in Mississippi~~. ~~To climb~~ is ~~some thing I truly appreciate doing~~. ~~Invoicing~~ is ~~my profession~~.<br><br>~~Feel~~ free to ~~surf~~ to ~~my page free psychic readings~~ ([~~http:~~//~~www~~.~~herandkingscounty~~.~~com~~/~~content~~/~~information~~-~~and~~-~~facts~~-~~you~~-~~must~~-~~know~~-~~about~~-~~hobbies~~ http://www.~~herandkingscounty~~.~~com~~/~~content~~/~~information~~-~~and~~-~~facts~~-~~you~~-~~must~~-~~know-about-hobbies~~])		In [[differential geometry]], a '''projective connection''' is a type of [[Cartan connection]] on a [[differentiable manifold]].

			The structure of a projective connection is modeled on the geometry of [[projective space]], rather than the [[affine space]] corresponding to an [[affine connection]]. Much like affine connections, though, projective connections also define [[geodesics]]. However, these geodesics are not [[affine parameter\|affinely parametrized]]. Rather they are projectively parametrized, meaning that their preferred class of parameterizations is acted upon by the group of [[fractional linear transformation]]s.

			Like an affine connection, projective connections have associated torsion and curvature.

			==Projective space as the model geometry==

			The first step in defining any Cartan connection is to consider the flat case: in which the connection corresponds to the [[Maurer-Cartan form]] on a [[homogeneous space]].

			In the projective setting, the underlying manifold ''M'' of the homogeneous space is the projective space '''RP'''<sup>n</sup> which we shall represent by [[homogeneous coordinates]] [''x''<sub>0</sub>,...,''x''<sub>n</sub>]. The symmetry group of ''M'' is ''G'' = PSL(''n''+1,'''R''').<ref>It is also possible to use PGL(''n''+1,'''R'''), but PSL(''n''+1,'''R''') is more convenient because it is connected.</ref> Let ''H'' be the [[isotropy group]] of the point [1,0,0,...,0]. Thus, ''M'' = ''G''/''H'' presents ''M'' as a homogeneous space.

			Let <math>{\mathfrak g}</math> be the [[Lie algebra]] of ''G'', and <math>{\mathfrak h}</math> that of ''H''. Note that <math>{\mathfrak g} = {\mathfrak s}{\mathfrak l}(n+1,{\mathbb R})</math>. As matrices relative to the homogeneous [[basis of a vector space\|basis]], <math>{\mathfrak g}</math> consists of [[trace-free]] (''n''+1)×(''n''+1) matrices:

			:<math>\left(
			\begin{matrix}
			\lambda&v^i\\
			w_j&a_j^i
			\end{matrix}
			\right),\quad
			(v^i)\in {\mathbb R}^{1\times n}, (w_j)\in {\mathbb R}^{n\times 1}, (a_j^i)\in {\mathbb R}^{n\times n}, \lambda = -\sum_i a_i^i
			</math>.

			And <math>{\mathfrak h}</math> consists of all these matrices with (''w''<sub>j</sub>) = 0. Relative to the matrix representation above, the Maurer-Cartan form of ''G'' is a system of 1-forms (ζ, α<sub>j</sub>, α<sub>j</sub><sup>i</sup>, α<sup>i</sup>) satisfying the structural equations<ref>Cartan's approach was to derive the structural equations from the volume-preserving condition on ''SL''(''n''+1) so that explicit reference to the Lie algebra was not required.</ref>
			:''d''ζ + ∑<sub>i</sub> α<sup>i</sup>∧α<sub>i</sub> = 0
			:''d''α<sub>j</sub> + α<sub>j</sub>∧ζ + ∑<sub>k</sub> α<sub>j</sub><sup>k</sup>∧α<sub>k</sub> = 0
			:''d''α<sub>j</sub><sup>i</sup> + α<sup>i</sup>∧α<sub>j</sub> + ∑<sub>k</sub> α<sub>k</sub><sup>i</sup>∧α<sub>j</sub><sup>k</sup> = 0
			:''d''α<sup>i</sup> + ζ∧α<sup>i</sup> + ∑<sub>k</sub>α<sup>k</sup>∧α<sub>k</sub><sup>i</sup> = 0<ref>A point of interest is this last equation is [[integrability condition\|completely integrable]], which means that the fibres of ''G'' → ''G''/''H'' can be defined using only the Maurer-Cartan form, by the [[Frobenius integration theorem]].</ref>

			==Projective structures on manifolds==

			A projective structure is a ''linear geometry'' on a manifold in which two nearby points are connected by a line (i.e., an unparametrized ''geodesic'') in a unique manner. Furthermore, an infinitesimal neighborhood of each point is equipped with a class of ''[[projective frame]]s''. According to Cartan (1924),
			:''Une variété (ou espace) à connexion projective est une variété numérique qui, au voisinage immédiat de chaque point, présente tous les caractères d'un espace projectif et douée de plus d'une loi permettant de raccorder en un seul espace projectif les deux petits morceaux qui entourent deux points infiniment voisins. ..''
			:''Analytiquement, on choisira, d'une manière d'ailleurs arbitraire, dans l'espace projectif attaché à chaque point '''a''' de la variété, un ''repére'' définissant un système de coordonnées projectives. .. Le raccord entre les espaces projectifs attachés à deux points infiniment voisins '''a''' et '''a'''' se traduira analytiquement par une transformation homographique. ..''<ref>A variety (or space) with projective connection is a numerical variety which, in the immediate neighbourhood of each point, possesses all the characters of a projective space and is moreover endowed with a law making it possible to connect in a single projective space the two small regions which surround two infinitely close points.

			Analytically, we choose, in a way otherwise arbitrary, a frame defining a projective frame of reference in the projective space attached to each point of the variety. .. The connection between the projective spaces attached to two infinitely close points '''a''' and '''a'''' will result analytically in a homographic (projective) transformation. ..</ref>

			This is analogous to Cartan's notion of an ''[[affine connection]]'', in which nearby points are thus connected and have an affine [[frame of reference]] which is transported from one to the other (Cartan, 1923):
			:''La variété sera dite à "connexion affine" lorsqu'on aura défini, d'une manière d'ailleurs arbitraire, une loi permettant de repérer l'un par rapport à l'autre les espaces affines attachés à deux points ''infiniment voisins'' quelconques '''m''' et '''m'''' de la variété; cete loi permettra de dire que tel point de l'espace affine attaché au point '''m'''' correspond à tel point de l'espace affine attaché au point '''m''', que tel vecteur du premier espace es parallèle ou équipollent à tel vecteur du second espace.''<ref>The variety will be said to "affinely connected" when one defines, in a way otherwise arbitrary, a law making it possible to place the affine spaces, attached to two arbitrary infinitely close points '''m''' and '''m'''' of the variety, in correspondence with each other; this law will make it possible to say that a particular point of the affine space attached to the point '''m'''' corresponds to a particular point of the affine space attached to the point '''m''', in such a way that a vector of the first space is parallel or equipollent with the corresponding vector of the second space.</ref>

			In modern language, a projective structure on an ''n''-manifold ''M'' is a [[Cartan geometry]] modelled on projective space, where the latter is viewed as a homogeneous space for PSL(''n''+1,'''R'''). In other words it is a PSL(''n''+1,'''R''')-bundle equipped with
			* a PSL(''n''+1,'''R''')-connection (the [[Cartan connection]])
			* a [[reduction of structure group]] to the stabilizer of a point in projective space
			such that the [[solder form]] induced by these data is an isomorphism.

			==Notes==

			<div class="references-small" style="-moz-column-count:2; column-count:2;">
			<references />
			</div>

			==References==

			* {{cite journal \| first = Élie \| last = Cartan \| title = Sur les variétés à connexion affine, et la théorie de la relativité généralisée (première partie) \| journal = Annales Scientifiques de l'École Normale Supérieure\| volume = 40 \| year = 1923 \| pages = 325–412 \| url=http://www.numdam.org/item?id=ASENS_1923_3_40__325_0}}
			* {{cite journal \| first = Élie \| last = Cartan \| title = Sur les varietes a connexion projective \| journal = Bulletin de la Société Mathématique \| volume = 52 \| year = 1924 \| pages = 205–241 \| url=http://www.numdam.org/item?id=BSMF_1924__52__205_0}}
			* Hermann, R., Appendix 1-3 in Cartan, E. ''Geometry of Riemannian Spaces'', Math Sci Press, Massachusetts, 1983.
			* {{cite journal \| first = Élie \| last = Cartan \| title = Espaces à connexion affine, projective et conforme \| journal = Acta Math. \| volume = 48 \| year = 1926 \| pages = 1–42 \| doi=10.1007/BF02629755}}
			* {{cite book \| first = R.W. \| last = Sharpe \| title = Differential Geometry: Cartan's Generalization of Klein's Erlangen Program \| publisher = Springer-Verlag, New York \| year = 1997 \| isbn = 0-387-94732-9}}

			==External links==
			*{{springer\|id=p/p075180\|title=Projective connection\|author=Ü. Lumiste}}

			[[Category:Differential geometry]]
			[[Category:Connection (mathematics)]]

en>Bgpaulus: /* Analog dividers */ replacing "fix" meta-template with "citation needed" template -- per Template:Fix documentation, "fix" should not be used directly on articles by itself

2012-08-12T22:19:43Z

Analog dividers: replacing "fix" meta-template with "citation needed" template -- per Template:Fix documentation, "fix" should not be used directly on articles by itself

New page

Andera is what you can contact her but she by no means truly favored that name. I've usually loved living in Mississippi. To climb is some thing I truly appreciate doing. Invoicing is my profession.<br><br>Feel free to surf to my page free psychic readings ([http://www.herandkingscounty.com/content/information-and-facts-you-must-know-about-hobbies http://www.herandkingscounty.com/content/information-and-facts-you-must-know-about-hobbies])

Frequency divider - Revision history

en>InverseHypercube: /* Digital dividers */

en>Bgpaulus: /* Analog dividers */ replacing "fix" meta-template with "citation needed" template -- per Template:Fix documentation, "fix" should not be used directly on articles by itself