Exercise · ZW-WELL-ORDERING-007 · ★★☆ Intermediate
Let \(B=\{n\in\mathbb{N} : n^2>20\}\) with the usual order. What is its least element?
Choose one option.
Show answer
- A. \(5\)
Explanation
Since \(4^2=16\) does not exceed 20 but \(5^2=25\) does, the least element is \(5\); well-ordering of \(\mathbb{N}\) guarantees that it exists.
Review the concept: Well-ordering
A set \(A\) is well-ordered by \(<\) if \(<\) is a total order on \(A\) and every nonempty subset of \(A\) has a least element.
\[\forall S\subseteq A(S\ne\varnothing\Rightarrow\exists m\in S\forall s\in S\,m\le s)\]
More practice problems: well-ordering
- ZW-WELL-ORDERING-001 Which of the following statements describes a well-ordered set?
- ZW-WELL-ORDERING-002 Is ℤ well-ordered by its usual order?
- ZW-WELL-ORDERING-003 Which of these sets is well-ordered by the usual order of ℝ?
- ZW-WELL-ORDERING-004 If A is well-ordered and B⊆ A is nonempty, what holds?
- ZW-WELL-ORDERING-005 Let A=\2,5,7\ with the usual order. What is its least element, and why is A well-ordered?
- ZW-WELL-ORDERING-006 Is the set \x∈ℚ : x>0\ well-ordered by its usual order?
- ZW-WELL-ORDERING-008 Which of these subsets of ℤ has a least element?
- ZW-WELL-ORDERING-009 Let A be a well-ordered set and f:A→ A such that f(a)<a for all a∈ A. What follows?