Discussion about math, puzzles, games and fun. Useful symbols: ÷ × ½ √ ∞ ≠ ≤ ≥ ≈ ⇒ ± ∈ Δ θ ∴ ∑ ∫ • π ƒ -¹ ² ³ °
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First I prove this:
Given there exist natural numbers such that .
Let be the smallest such natural number. Then
If we set , then . This proves existence.
For uniqueness, suppose with .
(i) Suppose .
Since is the least natural number such that we must have
. a contradiction.
(ii) If , then . Then
, another contradiction.
Hence, it must be that and ∴ .
Last edited by JaneFairfax (2009-01-21 22:21:53)
Now let , where . Then the mapping
is injective, by the previous result with and ,
Hence is a bijection from to a subset of – proving that the positive rationals are countable.
Then the negative rationals are also countable since the mapping is a bijection between and . Hence the rationals are countable.