In mathematics, an adherent point (also closure point or point of closure or contact point) of a subset A of a topological space X, is a point x in X such that every neighbourhood of x (i.e. every open set containing x) contains at least one point of A. A point x is an adherent point for A if and only if x is in the closure of A, thus
x
∈
Cl
(
A
)
⟺
∀
U
⊂
X
open
:
x
∈
U...
In mathematics, a subset
A
{\displaystyle A}
of a topological space is said to be dense-in-itself if
A
{\displaystyle A}
contains no isolated points.
Every dense-in-itself closed set is perfect. Conversely, every perfect set is dense-in-itself.
A simple example of a set which is dense-in-itself but not closed (and hence not a perfect set) is the subset of irrational numbers (considered as a subset of the real numbers). This set is dense-in-itself because every neighborhood of an irrational number...
This question is closed as being opinion-based. Perhaps I do not understand English well, could someone explain in more detail what opinion-based is on this question?
[N.B.: I use a Mac, but I'm assuming the following is not Mac-specific.]
I've noticed that, font-wise, SE sites can be divided into two categories:
1) Those that employ a sans-serif font (e.g., TeX SE, superuser SE, and this one). These display numerals as "lining figures" (~full-height number...
Description of the system
Assume two point masses one at the point $C$ and the other on the circumference of the circle with radius $R$. They are attracting one another gravitationally and no external forces are acting on them. The point mass at C has very large mass such that it is at the COM...
