The GATE Grind

GATE 2020 MA – Question 17

Topology · Connectedness and Compactness · 1 mark · Multiple choice

Let $U=\mathbb R^2\setminus\mathbb Q^2$ and $V=\mathbb R^2\setminus\{(x,y):x>0,y=1/x\}$ with Euclidean metric. Then

  1. Both U and V are disconnected.
  2. U is disconnected but V connected.
  3. U is connected but V disconnected.
  4. Both U and V are connected.

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Correct answer: (C) U is connected but V disconnected.

Explanation

Removing a countable set from the plane leaves it path connected: between any two remaining points choose an intermediate point avoiding the countably many lines that would make a joining segment hit a deleted point. Thus U is connected. The removed positive hyperbola separates V into the region $x>0,y>1/x$ and its complementary side, two nonempty relatively open parts. Hence V is disconnected and C holds.

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