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I am searching for the correct term for the following, if it exists.

A set $X\subset \mathbb{R}^2$ is called $r$-convex if for any two points $x_1, x_2\in X$ such that there exists an arc of radius $r$ connecting these two points, at least one arc lies in the set $X$.

Note 1: Obviously, there are always two arcs of radius $r$ (if one exists) connecting two points.

Note 2: When $r\rightarrow +\infty$ we have the definition of convex set.

My main interest is efficient algorithms for computation of convex hulls based on this definition of convexity.

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    $\begingroup$ I think I would disagree with your first note as, surely, if we take the set X to be a semicircle of radius r then there is only one arc of radius r between any two points. $\endgroup$
    – Stijn
    Commented Apr 18, 2014 at 12:56
  • $\begingroup$ @Stijn, I think he's saying that there are two arcs in $\mathbb{R}^2$, not necessarily in $X$. $\endgroup$
    – Dylan Thurston
    Commented Apr 18, 2014 at 14:20
  • $\begingroup$ It seems like a weird notion. Where does it come up? $\endgroup$
    – Dylan Thurston
    Commented Apr 18, 2014 at 14:20
  • $\begingroup$ Yes, I am talking about two arcs in $\mathbb{R}^2$. $\endgroup$
    – Fedor Nikitin
    Commented Apr 18, 2014 at 14:27
  • $\begingroup$ @Dylan At the moment I am working on algorithm for rounding sharp corners in polygon. Let $P$ be a set of points lying in polygon. I need to find $P^\prime$ such that its border consists of segments and arcs. Arc radius should not exceed $r$ and there are no sharp angles (less than $\pi$). I have guess that $P\prime = \mathbb{R}^2 \ co_r (\mathbb{R}^2 \ P)$, where $co_r$ is convex hull in the sense of definition above. My idea is to apply algorithms for regular convex hull with modified definition of convexity. $\endgroup$
    – Fedor Nikitin
    Commented Apr 18, 2014 at 14:40

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I can not comment posts because of too small reputation :-) It will change I hope.

In connection with usul's comment:

Also strongly convex sets are considered. Roughly speaking, one could require that together with two points the whole lens is contained in a set. For a definition see for instance here: http://arxiv.org/pdf/1207.4347.pdf

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