Joint probability distribution: Difference between revisions

Latest revision as of 19:11, 11 January 2015

Nike Shox Mens and the Kilby Hotel

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Joint probability distribution: Difference between revisions

Latest revision as of 19:11, 11 January 2015

Contents

Nike Shox Mens and the Kilby Hotel

A&f Singapore innovation and execution

A&f Singapore Hiring flexible

Nike Air Max CVPD

Jordan Shoes Singapore On WebMD

Navigation menu

@@ Line 1: / Line 1: @@
-{{See also|Envelope (waves)}}
+== Nike Shox Mens  and the Kilby Hotel ==
-[[Image:EnvelopeAnim.gif||right|thumb|500px|Construction of the envelope of a family of curves.]]
-In [[geometry]], an '''envelope''' of a [[Family of curves|family]] of [[curve]]s in the plane is a curve that is [[tangent]] to each member of the family at some point. Classically, a point on the envelope can be thought of as the intersection of two "adjacent" curves, meaning the limit of intersections of nearby curves. This idea can be generalized to an envelope of surfaces in space, and so on to higher dimensions.
-==Envelope of a family of curves==
+The Company encourages investors to consult this section of its website frequently for important information and news. Additionally, by subscribing to the business automatic email news release delivery, individuals can receive media directly from GMCR as it is released.Regarding Water For PeopleWater For People is really an international nonprofit organization specialized in providing access to improved water and sanitation systems as well as services in developing places. By working with local governments, the private sector, and partners, H2o For People ensures that everyone inside of a specific region has these basic needs met eternally. <br><br>So Ghent founded "Feel The Music,Inch an interactive workshop that gives free music lessons, tutored by professional musicians, to be able to children and teens who lost parents on 9/11. And when your woman met Pat and Kathy Gorman, it absolutely was [http://www.irodorijapaneserestaurant.com.sg/_old/mail/list.php?n=111-Nike-Shox-Mens Nike Shox Mens] perfect harmony. "We went out to New Jersey and met using them. <br><br>Clearwater Cave is the longest cave system in Southeast Most of asia. Although little of it can be open to park visitors without special permission, over a $ 100 or so kilometers of passages have been discovered so far. Not all of Mulu's limestone features are contained in caves. <br><br>  SEODigger report regarding domain Amante Amor Obssesso: S. E. PALMEIRAS Amor Vcio: Msica Amor Paixonite: [http://www.ashford.edu.sg/Chinese/define.php?a=68-Cheap-Abercrombie-Fitch-Fierce-Cologne Cheap Abercrombie Fitch Fierce Cologne] Filmes/Sries Amor Incondicional: Famlia Amor Inesquecvel: U amor prprio Toma um paracetamol p baixar a febre ourite usa Hexomedine, depois vai ao mdico n?. <br><br>"I congratulate my good friend Eric Nadel on winning the 2014 Ford C. Frick Award," Chief executive George W. Bush said in a statement. "The Alloy Electronic Network has marked this season with tremendous growth. Its One position for the second sequential year is truly testament to this incredible effort and excitement our [http://www.ashford.edu.sg/Chinese/define.php?a=91-A&f-Singapore A&f Singapore] digital team will continue to put forth in building a high quality and effective product," mentioned Matthew Diamond, CEO of Metal Media + Marketing. ClevverTV is the A single most watched entertainment media source for teens on the net, generating more than 30 million opinions per month on YouTube alone. <br><br>Above the next few weeks, Dr. Speller may examine challenges and [http://www.ashford.edu.sg/Chinese/define.php?a=47-Abercrombie-&-Fitch Abercrombie & Fitch] programs related to the preservation on the First Baptist Church (image, suitable) and the Kilby Hotel, and outline several paths that would result in the upkeep of both structures. Efficiency is important to the east Excessive Point neighborhood, as reports have documented that preservation includes a multiplier effect with construction jobs, infrastructure improvement, heritage travel related, and enriching the taxation base.<ul>
-Let each curve ''C''<sub>''t''</sub> in the family be given as the solution of an equation ''f''<sub>''t''</sub>(''x'',&nbsp;''y'')=0 (see [[implicit curve]]), where ''t'' is a parameter. Write ''F''(''t'',&nbsp;''x'',&nbsp;''y'')=''f''<sub>''t''</sub>(''x'',&nbsp;''y'') and assume ''F'' is differentiable.
+   <li>[http://f1023.org/index.php?title=User:Fxvwrvua#Nike_Air_Max_90_Singapore_Saudi_Arabia http://f1023.org/index.php?title=User:Fxvwrvua#Nike_Air_Max_90_Singapore_Saudi_Arabia]</li>
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+   <li>[http://www.dinglic.com/bbs/home.php?mod=space&uid=145240 http://www.dinglic.com/bbs/home.php?mod=space&uid=145240]</li>
+   <li>[http://www.proyectoalba.com.ar/spip.php?article66/&quot;/ http://www.proyectoalba.com.ar/spip.php?article66/&quot;/]</li>
+ </ul>
-The envelope of the family ''C''<sub>''t''</sub> is then defined as the set of points for which
+== A&f Singapore  innovation and execution ==
-:<math>F(t, x, y) = {\partial F \over \partial t}(t, x, y) = 0</math>
-for some value of ''t'',
-where <math>\partial F/\partial t</math> is the [[partial derivative]] of ''F'' with respect to ''t''.<ref>{{Citation |first=J. W. |last=Bruce |first2=P. J. |last2=Giblin |title=Curves and Singularities |publisher=Cambridge University Press |year=1984 |ISBN=0-521-42999-4}}</ref>
-Note that if ''t'' and ''u'', ''t''≠''u'' are two values of the parameter then the intersection of the curves ''C''<sub>''t''</sub> and ''C''<sub>''u''</sub> is given by
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-:<math>F(t, x, y) = F(u, x, y) = 0\,</math>
-or equivalently
+   <li>[http://auditionlive.com/activity/p/5981/ http://auditionlive.com/activity/p/5981/]</li>
-:<math>F(t, x, y) = \frac{F(u, x, y)-F(t, x, y)}{u-t} = 0.</math>
-Letting u→t gives the definition above.
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-An important special case is when ''F''(''t'',&nbsp;''x'',&nbsp;''y'') is a polynomial in ''t''. This includes, by clearing denominators, the case where ''F''(''t'',&nbsp;''x'',&nbsp;''y'') is a rational function in ''t''. In this case, the definition amounts to ''t'' being a double root of ''F''(''t'',&nbsp;''x'',&nbsp;''y''), so the equation of the envelope can be found by setting the [[discriminant]] of ''F'' to 0.
+== A&f Singapore Hiring  flexible ==
-For example, let ''C''<sub>''t''</sub> be the line whose ''x'' and ''y'' intercepts are ''t'' and 1−''t'', this is shown in the animation above. The equation of ''C''<sub>''t''</sub> is
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-:<math>\frac{x}{t}+\frac{y}{1-t}=1</math>
-or, clearing fractions,
+   <li>[http://www.proyectoalba.com.ar/spip.php?article66/ http://www.proyectoalba.com.ar/spip.php?article66/]</li>
-:<math>x(1-t)+yt-t(1-t)=t^2+(-x+y-1)t+x=0.\,</math>
-The equation of the envelope is then
+   <li>[http://lab.nqnwebs.com/lavoz_bak/spip.php?article5818/ http://lab.nqnwebs.com/lavoz_bak/spip.php?article5818/]</li>
-:<math>(-x+y-1)^2-4x=(x-y)^2-2(x+y)+1=0.\,</math>
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-Often when ''F'' is not a rational function of the parameter it may be reduced to this case by an appropriate substitution. For example if the family is given by ''C''<sub>θ</sub> with an equation of the form ''u''(''x'',&nbsp;''y'')cosθ+''v''(''x'',&nbsp;''y'')sinθ=''w''(''x'',&nbsp;''y''), then putting ''t''=''e''<sup>''i''θ</sup>, cosθ=(''t''+1/''t'')/2, sinθ=(''t''-1/''t'')/2''i'' changes the equation of the curve to
+== Nike Air Max CVPD ==
-:<math>u{1 \over 2}(t+{1\over t})+v{1 \over 2i}(t-{1\over t})=w</math>
-or
-:<math>(u-iv)t^2-2wt+(u+iv)=0.\,</math>
-The equation of the envelope is then given by setting the discriminant to 0:
-:<math>(u-iv)(u+iv)-w^2=0\,</math>
-or
-:<math>u^2+v^2=w^2.\,</math>
-===Alternative definitions===
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-# The envelope ''E''<sub>1</sub> is the limit of intersections of nearby curves ''C''<sub>''t''</sub>.
+== Jordan Shoes Singapore On WebMD ==
-# The envelope ''E''<sub>2</sub> is a curve tangent to all of the ''C''<sub>''t''</sub>.
-# The envelope ''E''<sub>3</sub> is the boundary of the region filled by the curves ''C''<sub>''t''</sub>.
-Then <math>E_1 \subseteq \mathcal{D}</math>, <math>E_2 \subseteq \mathcal{D}</math> and <math>E_3 \subseteq \mathcal{D}</math>, where <math>\mathcal{D}</math> is the set of curves given by the first definition at the beginning of this document.
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-==Examples==
+   <li>[http://214n.com/thread-290964-1-1.html http://214n.com/thread-290964-1-1.html]</li>
-===Example 1===
+   <li>[http://sgwto.com/bbs/forum.php?mod=viewthread&tid=810157 http://sgwto.com/bbs/forum.php?mod=viewthread&tid=810157]</li>
-These four definitions of the discriminant, ''E''<sub>1</sub>,''E''<sub>2</sub>, and ''E''<sub>3</sub>, can lead to different sets. Consider for instance the curve {{nowrap|1=''y'' = ''x''<sup>3</sup>}} parametrised by {{nowrap|γ : '''R''' → '''R'''<sup>2</sup>}} where {{nowrap|1=γ(''t'') = (''t'',''t''<sup>3</sup>)}}. The one-parameter family of curves will be given by the tangent lines to γ.
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-First we calculate the discriminant <math>\mathcal D</math>. The generating function is
-:<math> F(t,(x,y)) = 3t^2x - y - 2t^3.</math>
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-Calculating the partial derivative {{nowrap|1=''F''<sub>''t''</sub> = 6''t''(''x'' – ''t'')}}. It follows that either {{nowrap|1=''x'' = ''t''}} or {{nowrap|1=''t'' = 0}}. First assume that {{nowrap|1=''x'' = ''t'' and ''t'' ≠ 0}}. Substituting into F: <math>F(t,(t,y)) = t^3 - y \, </math>
-and so, assuming that ''t'' ≠ 0, it follows that {{nowrap|1=''F'' = ''F''<sub>''t''</sub> = 0}} if and only if {{nowrap|1=(''x'',''y'') = (''t'',''t''<sup>3</sup>)}}. Next, assuming that {{nowrap|1=''t'' = 0}} and substituting into ''F'' gives {{nowrap|1=''F''(0,(''x'',''y'')) = &minus;''y''}}.  So, assuming {{nowrap|1=''t'' = 0}}, it follows that {{nowrap|1=''F'' = ''F''<sub>''t''</sub> = 0}} if and only if {{nowrap|1=''y'' = 0}}. Thus the discriminant is the original curve and its tangent line at γ(0):
+   <li>[http://bbs.0714.cn/forum.php?mod=viewthread&tid=4121906 http://bbs.0714.cn/forum.php?mod=viewthread&tid=4121906]</li>
-:<math> \mathcal{D} = \{(x,y) \in \R^2 : y = x^3\} \cup \{(x,y) \in \R^2 : y = 0 \} \ . </math>
+ </ul>
-Next we calculate ''E''<sub>1</sub>. One curve is given by {{nowrap|1=''F''(''t'',(''x'',''y'')) = 0}} and a nearby curve is given by {{nowrap|''F''(''t'' + &epsilon;,(''x'',''y''))}} where ε is some very small number. The intersection point comes from looking at the limit of {{nowrap|1=''F''(''t'',(''x'',''y''))&nbsp;=&nbsp;''F''(''t'' + &epsilon;,(''x'',''y''))}} as ε tends to zero. Notice that {{nowrap|1=''F''(''t'',(''x'',''y''))&nbsp;=&nbsp;''F''(''t'' + &epsilon;,(''x'',''y''))}} if and only if
-:<math> L := F(t,(x,y)) - F(t+\varepsilon,(x,y)) = 2\varepsilon^3+6\varepsilon t^2+6\varepsilon^2t-(3\varepsilon^2+6\varepsilon t)x = 0. </math>
-If {{nowrap|1=''t'' ≠ 0}} then ''L'' has only a single factor of ε. Assuming that {{nowrap|1=''t'' ≠ 0}} then the intersection is given by
-:<math>\lim_{\varepsilon \to 0} \frac{1}{\varepsilon} L = 6t(t-x) \ . </math>
-Since {{nowrap|''t'' ≠ 0}} it follows that {{nowrap|1=''x'' = ''t''}}. The ''y'' value is calculated by knowing that this point must lie on a tangent line to the original curve γ: that {{nowrap|1=''F''(''t'',(''x'',''y'')) = 0}}. Substituting and solving gives ''y'' = ''t''<sup>3</sup>. When {{nowrap|1=''t'' = 0}}, ''L'' is divisible by ε<sup>2</sup>. Assuming that {{nowrap|1=''t'' = 0}} then the intersection is given by
-:<math>\lim_{\varepsilon \to 0} \frac{1}{\varepsilon^2} L = 3x \ . </math>
-It follows that {{nowrap|1=''x'' = 0}}, and knowing that {{nowrap|1=''F''(''t'',(''x'',''y'')) = 0}} gives {{nowrap|1=''y'' = 0}}. It follows that
-:<math> E_1 = \{(x,y) \in \R^2 : y = x^3 \} \ . </math>
-Next we calculate ''E''<sub>2</sub>. The curve itself is the curve that is tangent to all of its own tangent lines. It follows that
-:<math> E_2 = \{(x,y) \in \R^2 : y = x^3 \} \ . </math>
-Finally we calculate ''E''<sub>3</sub>.  Every point in the plane has at least one tangent line to γ passing through it, and so region filled by the tangent lines is the whole plane. The boundary ''E''<sub>3</sub> is therefore the empty set. Indeed, consider a point in the plane, say (''x''<sub>0</sub>,''y''<sub>0</sub>). This point lies on a tangent line if and only if there exists a ''t'' such that
-:<math>F(t,(x_0,y_0)) = 3t^2x_0 - y_0 - 2t^3 = 0 \ . </math>
-This is a cubic in ''t'' and as such has at least one real solution. It follows that at least one tangent line to γ must pass through any given point in the plane. If {{nowrap|''y'' > ''x''<sup>3</sup>}} and {{nowrap|''y'' > 0}} then each point (''x'',''y'') has exactly one tangent line to γ passing through it. The same is true if {{nowrap|''y'' < ''x''<sup>3</sup>}} {{nowrap|''y'' < 0}}. If {{nowrap|''y'' < ''x''<sup>3</sup>}} and {{nowrap|''y'' > 0}} then each point (''x'',''y'') has exactly three distinct tangent lines to γ passing through it. The same is true if {{nowrap|''y'' > ''x''<sup>3</sup>}} and {{nowrap|''y'' < 0}}. If {{nowrap|1=''y'' = ''x''<sup>3</sup>}} and {{nowrap|''y'' ≠ 0}} then each point (''x'',''y'') has exactly two tangent lines to γ passing through it (this corresponds to the cubic having one ordinary root and one repeated root). The same is true if {{nowrap|''y'' ≠ ''x''<sup>3</sup>}} and {{nowrap|1=''y'' = 0}}. If {{nowrap|1=''y'' = ''x''<sup>3</sup>}} and {{nowrap|1=''x'' = 0}}, i.e., {{nowrap|1=''x'' = ''y'' = 0}}, then this point has a single tangent line to γ passing through it (this corresponds to the cubic having one real root of multiplicity 3). It follows that
-:<math>E_3 = \varnothing. </math>
-===Example 2===
-In [[string art]] it is common to cross-connect two lines of equally spaced pins. What curve is formed?
-For simplicity, set the pins on the ''x''- and ''y''-axes; a non-[[orthogonal]] layout is a [[Coordinate rotation|rotation]] and [[scaling (geometry)|scaling]] away. A general straight-line thread connects the two points (0, ''k''&minus;''t'') and (''t'', 0), where ''k'' is an arbitrary scaling constant, and the family of lines is generated by varying the parameter ''t''. From simple geometry, the equation of this straight line is ''y'' = &minus;(''k''&nbsp;&minus;&nbsp;''t'')''x''/''t'' + ''k''&nbsp;&minus;&nbsp;''t''. Rearranging and casting in the form ''F''(''x'',''y'',''t'') = 0 gives:
-:{|cellpadding=0 cellspacing=0 border=0 width=100%
-|-
-| <math>F(x,y,t)=t^2 + t(y-x-k) + kx = 0\,</math>
-| style="text-align:right"|(1)
-|}
-Now differentiate ''F''(''x'',''y'',''t'') with respect to ''t'' and set the result equal to zero, to get
-:{|cellpadding=0 cellspacing=0 border=0 width=100%
-|-
-| <math>\frac{\partial F(x,y,t)}{\partial t}=2t+ y-x-k = 0\,</math>
-| style="text-align:right"|(2)
-|}
-These two equations jointly define the equation of the envelope. From (2) we have ''t'' = (&minus;''y'' + ''x'' + ''k'')/2. Substituting this value of ''t'' into (1) and simplifying gives an equation for the envelope in terms of ''x'' and ''y'' only:
-: <math>x^2 - 2xy + y^2 -2kx - 2ky + k^2 = 0\,</math>
-This is the familiar implicit [[conic section]] form, in this case a [[parabola]]. Parabolae remain parabolae under rotation and scaling; thus the string art forms a parabolic arc ("arc" since only a portion of the full parabola is produced). In this case an anticlockwise rotation through 45° gives the orthogonal parabolic equation ''y'' = ''x''<sup>2</sup>/(''k''√2) + ''k''/(2√2). Note that the final step of eliminating ''t'' may not always be possible to do analytically, depending on the form of ''F''(''x'',''y'',''t'').
-=== Example 3 ===
-Let ''I'' ⊂ '''R''' be an open interval and let γ : ''I'' → '''R'''<sup>2</sub> be a smooth plane curve parametrised by [[arc length]]. Consider the one-parameter family of normal lines to γ(''I''). A line is normal to γ at γ(''t'') if it passes through γ(''t'') and is perpendicular to the [[Differential geometry of curves#Tangent vector|tangent vector]] to γ at γ(''t''). Let '''T''' denote the unit tangent vector to γ and let '''N''' denote the unit [[Differential geometry of curves#Normal or curvature vector|normal vector]]. Using a dot to denote the [[dot product]], the generating family for the one-parameter family of normal lines is given by {{nowrap|1=''F'' : ''I'' &times; '''R'''<sup>2</sup> → '''R'''}} where
-:<math> F(t,{\bold x}) = ({\bold x} - \gamma(t)) \cdot {\bold T}(t) \ . </math>
-Clearly ('''x''' &minus; γ)·'''T''' = 0 if and only if '''x''' &minus; γ is perpendicular to '''T''', or equivalently, if and only if '''x''' &minus; γ is [[Parallel (geometry)|parallel]] to '''N''', or equivalently, if and only if '''x''' = γ + λ'''N''' for some λ ∈ '''R'''. It follows that
-:<math> L_{t_0} := \{ {\bold x} \in \R^2 : F(t_0,{\bold x}) = 0 \} </math>
-is exactly the normal line to γ at γ(''t''<sub>0</sub>). To find the discriminant of ''F'' we need to compute its partial derivative with respect to ''t'':
-:<math>  \frac{\partial F}{\partial t}(t,{\bold x}) = \kappa (t) ({\bold x}-\gamma(t))\cdot {\bold N}(t) - 1 \ , </math>
-where κ is the [[Curvature#One dimension in two dimensions: Curvature of plane curves|plane curve curvature]] of γ. It has been seen that ''F'' = 0 if and only if '''x''' - γ = λ'''N''' for some λ ∈ '''R'''. Assuming that ''F'' = 0 gives
-:<math> \frac{\partial F}{\partial t} = \lambda \kappa(t) - 1 \ . </math>
-Assuming that κ ≠ 0 it follows that λ = 1/κ and so
-:<math> \mathcal{D} = \gamma(t) + \frac{1}{\kappa(t)}{\bold N}(t) \ . </math>
-This is exactly the [[evolute]] of the curve γ.
-=== Example 4 ===
-[[File:Astroid as envelope.png|right|thumb|An [[astroid]] as the envelope of the family of lines connecting points (''s'',0), (0,''t'') with ''s''<sup>2</sup>&nbsp;+&nbsp;''t''<sup>2</sup>&nbsp;=&nbsp;1]]
-The following example shows that in some cases the envelope of a family of curves may be seen as the topologic boundary of a union of sets, whose boundaries are the curves of the envelope. For <math>s>0</math> and <math>t>0</math> consider the (open) right triangle in a Cartesian plane with vertices <math>(0,0)</math>, <math>(s,0)</math> and <math>(0,t)</math>
-:<math>T_{s,t}:=\left\{(x,y)\in\R_+^2:\ \frac{x}{s}+\frac{y}{t}<1\right\}.
-</math>
-Fix an exponent <math>\alpha>0</math>, and consider the union of all the triangles <math>T_{s,t} </math> subjected to the constraint <math>\textstyle s^\alpha+t^\alpha=1 </math>, that is the open set
-:<math>\Delta_\alpha:=\bigcup_ {s^\alpha+t^\alpha=1}  T_{s,t}.</math>
-To write a Cartesian representation for <math>\textstyle\Delta_\alpha</math>, start with any <math>\textstyle s>0</math>, <math>\textstyle t>0</math> satisfying <math>\textstyle s^\alpha+t^\alpha=1 </math> and any <math>\textstyle(x,y)\in\R_+^2</math>. The [[Hölder inequality#Notable special cases|Hölder inequality]] in <math>\textstyle\R^2</math> with respect to the conjugated exponents <math>p:=1+\frac{1}{\alpha}</math> and <math>\textstyle q:={1+\alpha}</math> gives:
-:<math>x^\frac{\alpha}{\alpha+1}+y^\frac{\alpha}{\alpha+1}\leq \left(\frac{x}{s}+\frac{y}{t}\right)^\frac{\alpha}{\alpha+1}\Big(s^\alpha+t^\alpha\Big)^\frac{1}{\alpha+1}=\left(\frac{x}{s}+\frac{y}{t}\right)^\frac{\alpha}{\alpha+1}</math>,
-with equality if and only if <math>\textstyle s:\,t=x^\frac{1}{1+\alpha}:\,y^\frac{1}{1+\alpha}</math>.
-In terms of a union of sets the latter inequality reads: the point <math>(x,y)\in\R_+^2</math> belongs to the set <math>\textstyle\Delta_\alpha</math>, that is, it belongs to some <math>\textstyle T_{s,t}</math> with <math>\textstyle s^\alpha+t^\alpha=1</math>, if and only if it satisfies
-:<math>x^\frac{\alpha}{\alpha+1}+y^\frac{\alpha}{\alpha+1}<1.</math>
-Moreover, the boundary in <math>\R_+^2</math> of the set <math>\textstyle \Delta_\alpha</math> is the envelope of the corresponding family of line segments
-:<math>\left\{(x,y)\in\R_+^2:\ \frac{x}{s}+\frac{y}{t}=1\right\}\ ,\qquad s^\alpha+t^\alpha=1</math>
-(that is, the hypotenuses of the triangles), and has Cartesian equation
-:<math>x^\frac{\alpha}{\alpha+1}+y^\frac{\alpha}{\alpha+1}=1.</math>
-Notice that, in particular, the value <math>\alpha=1</math> gives the arc of parabola of the example 1, and the value <math>\alpha=2</math> (meaning that all hypotenuses are unit length segments) gives the [[astroid]].
-==Envelope of a family of surfaces==
-A '''one-parameter family of surfaces''' in three-dimensional Euclidean space is given by a set of equations
-:<math>F(x,y,z,a)=0</math>
-depending on a real parameter ''a''.<ref>{{Citation |first=Luther P. |last=Eisenhart |title=A Treatise on the Differential Geometry of Curves and Surfaces |publisher=Schwarz Press |year=2008 |ISBN=1-4437-3160-9}}</ref> For example the tangent planes to a surface along a curve in the surface form such a family.
-Two surfaces corresponding to different values ''a'' and ''a' '' intersect in a common curve defined by
-:<math> F(x,y,z,a)=0,\,\,{F(x,y,z,a^\prime)-F(x,y,z,a)\over a^\prime -a}=0.</math>
-In the limit as ''a' '' approaches ''a'', this curve tends to a curve contained in the surface at ''a''
-:<math> F(x,y,z,a)=0,\,\,{\partial F\over \partial a}(x,y,z,a)=0.</math>
-This curve is called the '''characteristic''' of the family at ''a''. As ''a'' varies the locus of these characteristic curves defines a surface called the '''envelope''' of the family of surfaces.
-{{quotation|The envelope of a family of surfaces is tangent to each surface in the family along the characteristic curve in that surface.}}
-==Generalisations==
-The idea of an envelope of a family of smooth submanifolds follows naturally. In general, if we have a family of submanifolds with codimension ''c'' then we need to have at least a ''c''-parameter family of such submanifolds. For example: a one-parameter family of curves in three-space (''c'' = 2) does not, generically, have an envelope.
-==Applications==
-=== Ordinary differential equation ===
-Envelopes are connected to the study of [[ordinary differential equation]]s (ODEs), and in particular [[singular solution]]s of ODEs.<ref>{{Citation | last1=Forsyth | first1=Andrew Russell | title=Theory of differential equations | publisher=[[Dover Publications]] | location=New York | series=Six volumes bound as three | mr=0123757 | year=1959}}, §§100-106.</ref> Consider, for example, the one-parameter family of tangent lines to the parabola ''y'' = ''x''<sup>2</sup>. These are given by the generating family {{nowrap|1=''F''(''t'',(''x'',''y'')) = ''t''<sup>2</sup> – 2''tx'' + ''y''}}. The zero level set {{nowrap|1=''F''(''t''<sub>0</sub>,(''x'',''y'')) = 0}} gives the equation of the tangent line to the parabola at the point (''t''<sub>0</sub>,''t''<sub>0</sub><sup>2</sup>). The equation {{nowrap|1=''t''<sup>2</sup> – 2''tx'' + ''y'' = 0}} can always be solved for ''y'' as a function of ''x'' and so, consider
-:<math> t^2 - 2tx + y(x) = 0. \ </math>
-Substituting
-:<math> t = \left(\frac{dy}{dx}\right)/2 </math>
-gives the ODE
-:<math> \left(\frac{dy}{dx}\right)^2 \!\! - 4x\frac{dy}{dx} + 4y = 0. </math>
-Not surprisingly ''y''&nbsp;=&nbsp;2''tx''&nbsp;&minus;&nbsp;''t''<sup>2</sup> are all solutions to this ODE. However, the envelope of this one-parameter family of lines, which is the parabola ''y''&nbsp;=&nbsp;''x''<sup>2</sup>, is also a solution to this ODE. Another famous example is [[Clairaut's equation]].
-=== Partial differential equations ===
-Envelopes can be used to construct more complicated solutions of first order [[partial differential equation]]s (PDEs) from simpler ones.<ref>{{Citation | last1=Evans | first1=Lawrence C. | title=Partial differential equations | publisher=[[American Mathematical Society]] | location=Providence, R.I. | isbn=978-0-8218-0772-9 | year=1998}}.</ref>  Let ''F''(''x'',''u'',D''u'')&nbsp;=&nbsp;0 be a first order PDE, where ''x'' is a variable with values in an open set Ω&nbsp;⊂&nbsp;'''R'''<sup>''n''</sup>, ''u'' is an unknown real-valued function, D''u'' is the [[gradient]] of ''u'', and ''F'' is a continuously differentiable function that is regular in D''u''.  Suppose that ''u''(''x'';''a'') is an ''m''-parameter family of solutions:  that is, for each fixed ''a''&nbsp;∈&nbsp;''A''&nbsp;⊂&nbsp;'''R'''<sup>''m''</sup>, ''u''(''x'';''a'') is a solution of the differential equation.  A new solution of the differential equation can be constructed by first solving (if possible)
-:<math>D_a u(x;a) = 0\,</math>
-for ''a''&nbsp;=&nbsp;φ(''x'') as a function of ''x''.  The envelope of the family of functions {''u''(·,''a'')}<sub>''a''∈''A''</sub> is defined by
-:<math>v(x) = u(x;\varphi(x)),\quad x\in\Omega,</math>
-and also solves the differential equation (provided that it exists as a continuously differentiable function).
-Geometrically, the graph of ''v''(''x'') is everywhere tangent to the graph of some member of the family ''u''(''x'';''a''). Since the differential equation is first order, it only puts a condition on the tangent plane to the graph, so that any function everywhere tangent to a solution must also be a solution. The same idea underlies the solution of a first order equation as an integral of the [[Monge cone]].<ref>{{citation |first=Fritz |last=John |authorlink=Fritz John |title=Partial differential equations |publisher=Springer |edition=4th |year=1991 |isbn=978-0-387-90609-6}}.</ref>  The Monge cone is a cone field in the '''R'''<sup>''n''+1</sup> of the (''x'',''u'') variables cut out by the envelope of the tangent spaces to the first order PDE at each point. A solution of the PDE is then an envelope of the cone field.
-In [[Riemannian geometry]], if a smooth family of [[geodesic]]s through a point ''P'' in a [[Riemannian manifold]] has an envelope, then ''P'' has a [[conjugate point]] where any geodesic of the family intersects the envelope. The same is true more generally in the [[calculus of variations]]: if a family of extremals to a functional through a given point ''P'' has an envelope, then a point where an extremal intersects the envelope is a conjugate point to ''P''.
-=== Caustics ===
-[[Image:Circle caustic.png|thumb|Reflective caustic generated from a [[circle]] and parallel rays]]
-In [[geometrical optics]], a [[Caustic (optics)|caustic]] is the envelope of a family of [[light ray]]s. In this picture there is an [[circular arc|arc]] of a circle. The light rays (shown in blue) are coming from a source ''at infinity'', and so arrive parallel. When they hit the circular arc the light rays are scattered in different directions according to the [[Specular reflection|law of reflection]]. When a light ray hits the arc at a point the light will be reflected as though it had been reflected by the arc's [[tangent line]] at that point. The reflected light rays give a one-parameter family of lines in the plane. The envelope of these lines is the [[Caustic (optics)|reflective caustic]]. A reflective caustic will generically consist of [[smooth curve|smooth]] points and [[cusp (singularity)|ordinary cusp]] points.
-From the point of view of the calculus of variations, [[Fermat's principle]] (in its modern form) implies that light rays are the extremals for the length functional
-:<math>L[\gamma] = \int_a^b |\gamma'(t)|\,dt</math>
-among smooth curves γ on [''a'',''b''] with fixed endpoints γ(''a'') and γ(''b'').  The caustic determined by a given point ''P'' (in the image the point is at infinity) is the set of conjugate points to ''P''.<ref>{{Citation | last1=Born | first1=Max | author1-link=Max Born | title=Principle of Optics | publisher=[[Cambridge University Press]] | isbn=978-0-521-64222-4 | date=October 1999 }}, Appendix I: The calculus of variations.</ref>
-===Huygens's principle===
-Light may pass through anisotropic inhomogeneous media at different rates depending on the direction and starting position of a light ray.  The boundary of the set of points to which light can travel from a given point '''q''' after a time ''t'' is known as the [[wave front]] after time ''t'', denoted here by Φ<sub>'''q'''</sub>(''t'').  It consists of precisely the points that can be reached from '''q''' in time ''t'' by travelling at the speed of light.  [[Huygens–Fresnel principle|Huygens's principle]] asserts that the wave front set {{nowrap|&Phi;<sub>'''q'''<sub>0</sub></sub>(''s'' + ''t'')}} is the envelope of the family of wave fronts {{nowrap|&Phi;<sub>'''q'''</sub>(''s'')}} for '''q'''&nbsp;∈&nbsp;Φ<sub>'''q'''<sub>0</sub></sub>(''t''). More generally, the point '''q'''<sub>0</sub> could be replaced by any curve, surface or closed set in space.<ref>{{Citation | last1=Arnold | first1=V. I. | author1-link=Vladimir Arnold | title=Mathematical Methods of Classical Mechanics, 2nd ed. | publisher=[[Springer-Verlag]] | location=Berlin, New York | isbn=978-0-387-96890-2; ISBN 978-0-387-96890-2 | year=1997}}, §46.</ref>
-==See also==
-* [[Ruled surface]]
-* [[Caustic (mathematics)]]
-==References==
-{{Reflist}}
-==External links==
-*{{MathWorld|title=Envelope|urlname=Envelope}}
-*[http://mathcurve.com/courbes2d/enveloppe/enveloppe.shtml "Enveloppe d'une Famille de Courbes Planes" at Encyclopédie des Formes Mathématiques Remarquables]
-{{Differential transforms of plane curves}}
-{{DEFAULTSORT:Envelope (Mathematics)}}
-[[Category:Differential geometry]]
-[[Category:Analytic geometry]]

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