Exercise · ZW-MEASURABLE-FUNCTION-006 · ★★☆ Intermediate
Let \(f(x)=\begin{cases}1 & x\ge 0\\ 0 & x<0\end{cases}\) on ℝ. What is \(f^{-1}((1/2,\infty))\)?
Choose one option.
Show answer
- C. \([0,\infty)\)
Explanation
Since f takes the value 1 only for x at least 0, the preimage of a threshold between 0 and 1 is the half-line closed at 0. Taking the open half-line omits the point 0.
Review the concept: Measurable function
A function f: X → ℝ is measurable with respect to a σ-algebra \(\mathcal{A}\) on X if the preimage \(f^{-1}(B)\) belongs to \(\mathcal{A}\) for every Borel set B of ℝ.
\[f^{-1}(B)\in\mathcal{A}\ \forall B\in\mathcal{B}(\mathbb{R})\]
More practice problems: measurable function
- ZW-MEASURABLE-FUNCTION-001 What is the name of the condition satisfied by f when f^-1(B) lies in A for every Borel B?
- ZW-MEASURABLE-FUNCTION-002 Which σ-algebra is usually used on ℝ to define measurability of real functions?
- ZW-MEASURABLE-FUNCTION-003 Which of these functions from ℝ to ℝ is NOT measurable with respect to the Borel σ-algebra?
- ZW-MEASURABLE-FUNCTION-004 What kind of set is the preimage f^-1((a,∞)) when f is measurable?
- ZW-MEASURABLE-FUNCTION-005 Is every continuous function from ℝ to ℝ measurable with respect to the Borel σ-algebra?
- ZW-MEASURABLE-FUNCTION-007 Let f_n(x)=x^n on [0,1]. What is the pointwise limit f and why is it measurable?
- ZW-MEASURABLE-FUNCTION-008 If f and g are measurable, why is f+g measurable?
- ZW-MEASURABLE-FUNCTION-009 Let f be measurable with f^-1((a,∞))inA for all a∈ℝ. What does this imply about f^-1([a,∞))?