Question

Let a be a countable subset of r which is well-ordered with respect to the usual ordering on r (where well-ordered means that every nonempty subset has a minimum element in it). Then a has an order preserving bijection with a subset of n.

Answer 1

Let us define an ordinality to be an order- isomorphism class of well-ordered ... Example 1.2.3: The usual ordering on N is a well- ordering. ... Proof: Let Y be a nonempty subset of X+.

Puzzle 6: Solution - Harvard Math ...