If you're not in a union, you're in none; if not in an intersection, you're not in all -- you might not be in one!
- A1: + and · are closed binary operations on the reals.
- A2: + and · are associative.
- A3: + and · are commutative.
- A4: Distributivity holds: .
- A5: ∃ identities: and .
- A6: ∃ additive inverses.
- A7: ∃ multiplicative inverses (for ).
- A8: ∃ non-empty subset P ∈ IR such that the following hold:
- a, b ∈ P → a + b ∈ P
- a, b ∈ P → a · b ∈ P
- a ∈ IR → (a ∈ P) ∨ (−a ∈ P) ∨ (a = 0)
- A9: the reals are complete.
Exercise 9, p. 34 hint:
Now everything in the sum multiplied by in the final term can be bounded above. For example, we can always demand that , and we have an upper bound (call it ) on . So we can assert that
where is just some number, and must be chosen to be less than 1.
Now choose appropriately.
Some comments on proofs
Watch for some of these problems:
- Assuming the theorem that you're in the process of proving.
- Assuming that the "arbitrary" sets you're dealing with are denumerable, or even finite.
- Forgetting to prove an "iff" proof in both directions.