<p>We introduce a generalization of the Tukey reducibility, which we call the <i>pre-Tukey reducibility</i>. While the basics of the original Tukey reducibility heavily rely on the axiom of choice, the pre-Tukey reducibility works well in <InlineEquation ID="IEq4"> <EquationSource Format="TEX">\(\textsf{ZF}\)</EquationSource> <EquationSource Format="MATHML"><math> <mi mathvariant="sans-serif">ZF</mi> </math></EquationSource> </InlineEquation> (without the axiom of choice) to compare cofinal types of directed sets. In <InlineEquation ID="IEq5"> <EquationSource Format="TEX">\(\textsf{ZF}\)</EquationSource> <EquationSource Format="MATHML"><math> <mi mathvariant="sans-serif">ZF</mi> </math></EquationSource> </InlineEquation>, we show that two directed sets are pre-Tukey equivalent if and only if there exists a directed set into which both directed sets can be cofinally embedded. In this paper, we investigate the pre-Tukey reducibility between <InlineEquation ID="IEq6"> <EquationSource Format="TEX">\(\sigma \)</EquationSource> <EquationSource Format="MATHML"><math> <mi>σ</mi> </math></EquationSource> </InlineEquation>-directed sets <InlineEquation ID="IEq7"> <EquationSource Format="TEX">\((\omega ^\omega , \le ^*), (\mathcal {M}, \subseteq ), (\mathcal {N}, \subseteq ), ([\omega ^\omega ]^\omega , \subseteq ) \)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mrow> <mo stretchy="false">(</mo> <msup> <mi>ω</mi> <mi>ω</mi> </msup> <mo>,</mo> <msup> <mo>≤</mo> <mo>∗</mo> </msup> <mo stretchy="false">)</mo> </mrow> <mo>,</mo> <mrow> <mo stretchy="false">(</mo> <mi mathvariant="script">M</mi> <mo>,</mo> <mo>⊆</mo> <mo stretchy="false">)</mo> </mrow> <mo>,</mo> <mrow> <mo stretchy="false">(</mo> <mi mathvariant="script">N</mi> <mo>,</mo> <mo>⊆</mo> <mo stretchy="false">)</mo> </mrow> <mo>,</mo> <mrow> <mo stretchy="false">(</mo> <msup> <mrow> <mo stretchy="false">[</mo> <msup> <mi>ω</mi> <mi>ω</mi> </msup> <mo stretchy="false">]</mo> </mrow> <mi>ω</mi> </msup> <mo>,</mo> <mo>⊆</mo> <mo stretchy="false">)</mo> </mrow> </mrow> </math></EquationSource> </InlineEquation> and <InlineEquation ID="IEq8"> <EquationSource Format="TEX">\((\omega _1, \le )\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mo stretchy="false">(</mo> <msub> <mi>ω</mi> <mn>1</mn> </msub> <mo>,</mo> <mo>≤</mo> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation> where <InlineEquation ID="IEq9"> <EquationSource Format="TEX">\(\le ^*\)</EquationSource> <EquationSource Format="MATHML"><math> <msup> <mo>≤</mo> <mo>∗</mo> </msup> </math></EquationSource> </InlineEquation> denotes the dominating relation on <InlineEquation ID="IEq10"> <EquationSource Format="TEX">\(\omega ^\omega \)</EquationSource> <EquationSource Format="MATHML"><math> <msup> <mi>ω</mi> <mi>ω</mi> </msup> </math></EquationSource> </InlineEquation>, and <InlineEquation ID="IEq11"> <EquationSource Format="TEX">\(\mathcal {M} \)</EquationSource> <EquationSource Format="MATHML"><math> <mi mathvariant="script">M</mi> </math></EquationSource> </InlineEquation> and <InlineEquation ID="IEq12"> <EquationSource Format="TEX">\(\mathcal {N}\)</EquationSource> <EquationSource Format="MATHML"><math> <mi mathvariant="script">N</mi> </math></EquationSource> </InlineEquation> denote the ideal of meager and null sets of <InlineEquation ID="IEq13"> <EquationSource Format="TEX">\(2^\omega \)</EquationSource> <EquationSource Format="MATHML"><math> <msup> <mn>2</mn> <mi>ω</mi> </msup> </math></EquationSource> </InlineEquation>, respectively. We show that under <InlineEquation ID="IEq14"> <EquationSource Format="TEX">\(\textsf{ZF}+ \textsf{DC}\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi mathvariant="sans-serif">ZF</mi> <mo>+</mo> <mi mathvariant="sans-serif">DC</mi> </mrow> </math></EquationSource> </InlineEquation> and certain assumptions on sets of reals, these directed sets have pairwise distinct cofinal types. The assumptions we consider hold in the Solovay model and in <InlineEquation ID="IEq15"> <EquationSource Format="TEX">\(L(\mathbb {R})\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>L</mi> <mo stretchy="false">(</mo> <mi mathvariant="double-struck">R</mi> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation> satisfying the axiom of determinacy.</p>

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Generalized Tukey reducibility between \(\sigma \)-directed sets

  • Hiroshi Sakai,
  • Toshimasa Tanno

摘要

We introduce a generalization of the Tukey reducibility, which we call the pre-Tukey reducibility. While the basics of the original Tukey reducibility heavily rely on the axiom of choice, the pre-Tukey reducibility works well in \(\textsf{ZF}\) ZF (without the axiom of choice) to compare cofinal types of directed sets. In \(\textsf{ZF}\) ZF , we show that two directed sets are pre-Tukey equivalent if and only if there exists a directed set into which both directed sets can be cofinally embedded. In this paper, we investigate the pre-Tukey reducibility between \(\sigma \) σ -directed sets \((\omega ^\omega , \le ^*), (\mathcal {M}, \subseteq ), (\mathcal {N}, \subseteq ), ([\omega ^\omega ]^\omega , \subseteq ) \) ( ω ω , ) , ( M , ) , ( N , ) , ( [ ω ω ] ω , ) and \((\omega _1, \le )\) ( ω 1 , ) where \(\le ^*\) denotes the dominating relation on \(\omega ^\omega \) ω ω , and \(\mathcal {M} \) M and \(\mathcal {N}\) N denote the ideal of meager and null sets of \(2^\omega \) 2 ω , respectively. We show that under \(\textsf{ZF}+ \textsf{DC}\) ZF + DC and certain assumptions on sets of reals, these directed sets have pairwise distinct cofinal types. The assumptions we consider hold in the Solovay model and in \(L(\mathbb {R})\) L ( R ) satisfying the axiom of determinacy.