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Milner–Rado paradox
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Milner–Rado paradox : ウィキペディア英語版
Milner–Rado paradox
In set theory, a branch of mathematics, the Milner – Rado paradox, found by , states that every ordinal number α less than the successor ''κ''+ of some cardinal number κ can be written as the union of sets ''X''1,''X''2,... where ''X''''n'' is of order type at most ''κ''''n'' for ''n'' a positive integer.
==Proof==
The proof is by transfinite induction. Let ''\alpha'' be a limit ordinal (the induction is trivial for successor ordinals), and for each ''\beta<\alpha'', let ''\_n'' be a partition of ''\beta'' satisfying the requirements of the theorem.
Fix an increasing sequence \_ cofinal in \alpha with \beta_0=0.
Note \mathrm\,(\alpha)\le\kappa.
Define:
:X^\alpha _0 = \;\ \ X^\alpha_ = \bigcup_\gamma X^X^\alpha_n = \bigcup _n \bigcup _\gamma X^}_n\setminus \beta_\gamma = \bigcup_\gamma \beta_\setminus \beta_\gamma = \alpha \setminus \beta_0
and so ''\bigcup_nX^\alpha_n = \alpha''.
Let \mathrm\,(A) be the order type of ''A''. As for the order types, clearly \mathrm(X^\alpha_0) = 1 = \kappa^0.
Noting that the sets \beta_\setminus\beta_\gamma form a consecutive sequence of ordinal intervals, and that each X^}_n we get that:
:\mathrm(X^\alpha_) = \sum_\gamma \mathrm(X^(\alpha) \leq \kappa^n\cdot\kappa = \kappa^

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