A boundary is that which is the extremity of something.
Let \(c\) be a figure of a plane \(p\)^{1}. A point \(A\) is called a boundary point of \(c\), if every^{2} circle with the centre at \(A\) and a radius \(r > 0\) has points in common with the figure \(c\) and with \(p\setminus c\) (i.e. the everything in the plane \(p\) except the figure \(c\):
The set of all boundary points of \(c\) is called the boundary of the figure \(c\) and denoted by \(\delta c\).
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Propositions: 4 5