Let H be the set of all points (x, y) in ℝ2 such that x2 + xy + 3y2 = 3. Show that H is a closed subset of ℝ2 (considered with the Euclidean metric). Is H bounded?

Advanced Engineering Mathematics
10th Edition
ISBN:9780470458365
Author:Erwin Kreyszig
Publisher:Erwin Kreyszig
Chapter2: Second-order Linear Odes
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Let H be the set of all points (xy) in ℝ2 such that x2 + xy + 3y2 = 3. Show that H is a closed subset of ℝ2 (considered with the Euclidean metric). Is H bounded?

Bear in mind I can't use facts about continous functions. So this is what I learned in this chapter:

  • Balls/neighborhood centered at a point a consists of all points strictly within some radius r of the point.
  • A set is open if every point in the set has a small ball around it that is contained in the set.
  • A set is closed if its complement is open, and sequentially closed if every sequence in the set has all accumulation points in the set.  We proved these are equivalent.
  • A set is sequentially compact if every sequence in the set has accumulation points in the set.
  • A set is compact if every cover by open sets has a finite subcover.  Compact sets are sequentially compact.
  • In the reals, compact, sequentially compact, and closed + bounded are equivalent.
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