Zentralblatt MATH
Publications of (and about) Paul Erdös
Zbl.No: 111.01201
Autor: Erdös, Pál; Hajnal, András
Title: On a classification of denumerable order types and an application to the partition calculus (In English)
Source: Fundam. Math. 51, 117-129 (1962).
Review: Let \Theta and \Theta' be denumerable order types; \Theta is discrete if \eta \not \leq \Theta. It is shown that if \Theta is discrete, \Theta has a rank \rho (\Theta) (an ordinal < \omega1) defined from the way \Theta is attainable from 0 and 1 via a transfinite process of \omega- and \omega^*-additions. It is shown that if \Theta is not discrete, \Theta is a sum of type \eta, 1+\eta, \eta+1, or 1+\eta+1 of non-zero discrete types. Among the theorems (here paraphrased) in the partition calculus proved by using rank are the following statements (the bracketed insertions have been made by the reviewer). \Theta > (\Theta, \aleph0)2 if (and only if) \Theta = \omega or \Theta = \omega^* or \eta \leq \Theta [or \Theta < 2]. \Theta (not)> (\Theta',\aleph0)2 if \Theta is discrete and \Theta'\ne n+\omega^* and \Theta' \ne \omega+n for each n < \omega [and \Theta' is infinite]. \Theta > (\omega+n,\aleph0)2 if and only if \omega · \omega^* \leq \Theta.
[Minor errors: On line 27 of p. 125 replace "\overline{\overline{S'' · Sn'0}} = \aleph0" by "either both n0 < n0' and \overline {\overline {S'' · Sn'0}} = \aleph". Lines 18-20 of p. 125 neglect the possibility that \overline{\overline{S'}} = \aleph0 and [S']2 \subset I2; however, this possibility may be handled trivially.]
Reviewer: A.H.Kruse
Classif.: * 05D10 Ramsey theory
04A10 Ordinal and cardinal numbers; generalizations
04A20 Combinatorial set theory
03E05 Combinatorial set theory (logic)
Index Words: set theory
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