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The interval (−1, 1) is homeomorphic to :

  • A.The parabola \(y = x^{2}\)
  • B.The ellipse \(\frac{x^{2}}{4} + \frac{y^{2}}{9} = 1\)
  • C.The circle \(x^{2} + y^{2} = 1\)
  • D.The interval [−1, 1]

Answer: A

(−1, 1) is homeomorphic to the parabola \(y = x^{2}\).

The parabola is the graph of a continuous function on R, so \(x \mapsto (x, x^{2})\) is a homeomorphism from R onto it. And (−1, 1) is homeomorphic to R, for example by \(x \mapsto \tan\frac{\pi x}{2}\).

The others are distinguished by topological invariants. The ellipse and the circle are compact, while (−1, 1) is not. [−1, 1] is compact too. Removing a point from the circle or ellipse leaves it connected, but removing a point from an open interval disconnects it.

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