Counting Ordered Pairs
The set of ordered pairs of elements of a countable set is countable.
Proof
Every integer is uniquely represented in the form 2pq, where
For a pair (m, n) ∈ N×N, where N is the set of natural numbers, define
f(m, n) = 2m - 1(2n - 1).
Function f is a bijection from N×N to N. (It is obviously 1-1. It is onto because of the sentence that opens the proof.)
That's it.
References
- DAVID M. BRADLEY, Counting Ordered Pairs, Math magazine, 83 (2010) 302
Countability of Rational Numbers
- Counting Ordered Pairs
- Countable Times Countable Is Countable
- Countability of Rational Numbers
- Countability of Rational Numbers: PWW
- Countability Principle
- Countability of Rational Numbers via Conitnued Fractions
- Fractions on a Binary Tree II
- Countability of Rational Numbers as a Union of Finite Sets
- A New Proof That the Rationals Are Countable
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