Machine-translated by Google. Questions are authored in English.
Top

Discussion

Let L be a countable subset of \(\mathbf{R}\) and M be a Lebesgue measurable subset of \(\mathbf{R}\) with \(m(M) \gt 0\). If \(N = L \cup M\) and \(O = (L \cup M) \backslash (L \cap M)\), then :

  • A.\(m(M) = m(N) = m(O)\)
  • B.\(m(M) \lt m(N) = m(O)\)
  • C.\(m(M) = m(N) \lt m(O)\)
  • D.\(m(M) \lt m(N) \lt m(O)\)

Answer: A

The answer is \(m(M) = m(N) = m(O)\).

A countable set has Lebesgue measure 0, so L and every subset of it are null. N differs from M only by \(L \backslash M\), a null set, so \(m(N) = m(M)\).

O is the symmetric difference of L and M. It differs from M by removing \(L \cap M\) and adding \(L \backslash M\), both null, so \(m(O) = m(M)\) as well.

Adding or removing countably many points never changes Lebesgue measure, so no strict inequality is possible, which rules out the other three options.

No comment is present. Be the first to comment.
Loading…

Post your comment

Please log in or register to post a comment.