We present a bound for the weak Lindelöf number of the Gδ-modification of a Hausdorff space which implies various known cardinal inequalities, including the following two fundamental results in the theory of cardinal invariants in topology: |X|≤2^{L(X)χ(X)} (Arhangel'skii) and |X|≤2^{c(X)χ(X)} (Hajnal-Juhasz). This solves a question that goes back to Bell, Ginsburg and Woods and is mentioned in Hodel's survey on Arhangel'skii's Theorem. In contrast to previous attempts we do not need any separation axiom beyond T2.
Angelo Bella, Santi Spadaro (2020). A common extension of Arhangel'skii's Theorem and the Hajnal-Juhasz inequality. CANADIAN MATHEMATICAL BULLETIN, 63(1), 197-203 [10.4153/S0008439519000420].
A common extension of Arhangel'skii's Theorem and the Hajnal-Juhasz inequality
Santi Spadaro
2020-01-01
Abstract
We present a bound for the weak Lindelöf number of the Gδ-modification of a Hausdorff space which implies various known cardinal inequalities, including the following two fundamental results in the theory of cardinal invariants in topology: |X|≤2^{L(X)χ(X)} (Arhangel'skii) and |X|≤2^{c(X)χ(X)} (Hajnal-Juhasz). This solves a question that goes back to Bell, Ginsburg and Woods and is mentioned in Hodel's survey on Arhangel'skii's Theorem. In contrast to previous attempts we do not need any separation axiom beyond T2.
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