From Vectorcalcumb

Contents 1 Topology 1.1 What is a topology on a set? 1.2 The "ball" topology on Rn 1.3 The "box" topology on Rn

Topology

What is a topology on a set?

Open sets. 

Sets which do not intersect their boundary, that is if
<m>A \\subset \\R ^n</m> then A is open if <m> A = int(A)</m>

Closed sets. 

A set <m> A </m> is said to be closed if it's complement <m>R^{n} - A</m> is open.

• • Neighborhood of a point.

Interior points of a set

<m>P \\in int(A) </m> if there exists a Ball <m>B</m> centered at <m>P</m> with radius >0 s.t. <m>B \\subset A</m>

[AW: note/question about this on the discussion page]

• • Exterior point of a set. Exterior of a set.
  • Boundary point of a set. Boundary of a set.
  • Compact sets.
  • Acumulation point of a set.

The "ball" topology on Rⁿ

• • The norm-2 on Rⁿ.
  • Open balls in norm-2.
  • Open sets in the norm-2 topology.

The "box" topology on Rⁿ

• • The norm-infinity on Rⁿ.
  • Open balls in norm-infinity.
  • Open sets in the norm-infinity topology.
• Fact: The "ball" topology on Rⁿ is the same as the "box" topology on Rⁿ
• Induced topology on a subset of Rⁿ