View source for Contingent

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''of a subset $E$ of a Euclidean space at a point $a\in E$''

The union of rays $\overrightarrow{ab}$ with origin $a$ for which there exists a sequence of points $b_n\in E$ converging to $a$ such that the sequence of rays $\overrightarrow{ab_n}$ converges to $\overrightarrow{ab}$. It is denoted by $\operatorname{contg}(E,a)$. For an $m$-dimensional differentiable manifold $E$, $\operatorname{contg}(E,a)$ is the same as the $m$-dimensional tangent plane to $E$ at $a$. This concept proves useful in the study of differentiability properties of functions. If for every point $a$ of a set $E$ in the plane, $\operatorname{contg}(E,a)$ is not the whole plane, then $E$ can be partitioned into a countable number of parts situated on rectifiable curves. This theorem has been repeatedly generalized and refined. For example, a set of finite Hausdorff $p$-measure, $p=1,\dots,n-1$, located in an $n$-dimensional Euclidean space partitions into a countable number of parts, one of which has zero Favard measure of order $p$, while each of the remaining parts is situated on some Lipschitz surface of dimension $p$; for almost-all $x\in E$ (in the sense of the Hausdorff $p$-measure), $\operatorname{contg}(E,a)$ is a plane of dimension $p$ if all variations of the set $E$ are finite and, beginning with the $(p+1)$-th, vanish.


====Comments====
More on contingents (and the related notion of paratingent) can be found in G. Choquet's monograph [[#References|[a1]]]. Contingents are useful in optimization problems nowadays.

====References====
<table>
<TR><TD valign="top">[1]</TD> <TD valign="top">  G. Bouligand,   "Introduction à la géométrie infinitésimale directe" , Vuibert  (1932)</TD></TR><TR><TD valign="top">[2]</TD> <TD valign="top">  S. Saks,   "Theory of the integral" , Hafner  (1952)  (Translated from French)</TD></TR><TR><TD valign="top">[3]</TD> <TD valign="top">  H. Federer,   "Geometric measure theory" , Springer  (1969)</TD></TR><TR><TD valign="top">[4]</TD> <TD valign="top">  L.D. Ivanov,   "Variations of sets and functions" , Moscow  (1975)  (In Russian)</TD></TR>
<TR><TD valign="top">[a1]</TD> <TD valign="top">  G. Choquet,   "Outils topologiques et métriques de l'analyse mathématique" , Centre Docum. Univ. Paris  (1969)  (Rédigé par C. Mayer)</TD></TR>
<TR><TD valign="top">[a2]</TD> <TD valign="top">  J.P. Aubin,   I. Ekeland,   "Applied nonlinear analysis" , Wiley (Interscience)  (1984)</TD></TR>
</table>