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Exercise 1562Logic, sets and proofs › Countability and cardinalityM1Added 3 Nov 2026

Ordinals vs cardinals

#ordinals#cardinals#distinction

Statement

Explain the difference between ordinals and cardinals.

Hint

Ordinals measure order type; cardinals measure size.

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Solution

Ordinals. Von Neumann ordinals: each ordinal is the set of all smaller ordinals. ω={0,1,2,…}\omega=\{0,1,2,\ldots\} is the first infinite ordinal; ω+1\omega+1, ω⋅2\omega\cdot2, ω2\omega^2, ωω\omega^\omega are further ordinals.

Cardinals. Cardinals measure cardinality; ℵ0=∣ω∣\aleph_0=|\omega|, ℵ1\aleph_1 is the first uncountable cardinal.

Distinction. ω\omega and ω+1\omega+1 have the same cardinality (ℵ0\aleph_0) but different order types.

Conclusion. Ordinals refine cardinals with order structure.