<p>For a locally compact group <InlineEquation ID="IEq1"> <EquationSource Format="TEX">\(G\)</EquationSource> <EquationSource Format="MATHML"><math> <mi>G</mi> </math></EquationSource> </InlineEquation>, let <InlineEquation ID="IEq2"> <EquationSource Format="TEX">\( AP (G)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>A</mi> <mi>P</mi> <mo stretchy="false">(</mo> <mi>G</mi> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation> and <InlineEquation ID="IEq3"> <EquationSource Format="TEX">\( WAP (G)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>W</mi> <mi>A</mi> <mi>P</mi> <mo stretchy="false">(</mo> <mi>G</mi> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation> be respectively the <InlineEquation ID="IEq4"> <EquationSource Format="TEX">\(C^{*}\)</EquationSource> <EquationSource Format="MATHML"><math> <mmultiscripts> <mi>C</mi> <mrow /> <mrow> <mrow /> <mo>∗</mo> </mrow> </mmultiscripts> </math></EquationSource> </InlineEquation>-algebras of almost periodic and weakly almost periodic functions on <InlineEquation ID="IEq5"> <EquationSource Format="TEX">\(G\)</EquationSource> <EquationSource Format="MATHML"><math> <mi>G</mi> </math></EquationSource> </InlineEquation>. For a bounded continuous function <InlineEquation ID="IEq6"> <EquationSource Format="TEX">\(f\)</EquationSource> <EquationSource Format="MATHML"><math> <mi>f</mi> </math></EquationSource> </InlineEquation> on <InlineEquation ID="IEq7"> <EquationSource Format="TEX">\(G\)</EquationSource> <EquationSource Format="MATHML"><math> <mi>G</mi> </math></EquationSource> </InlineEquation>, <InlineEquation ID="IEq8"> <EquationSource Format="TEX">\(f\)</EquationSource> <EquationSource Format="MATHML"><math> <mi>f</mi> </math></EquationSource> </InlineEquation> is said to be strictly w.a.p. if its double orbit <InlineEquation ID="IEq9"> <EquationSource Format="TEX">\(O(f)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>O</mi> <mo stretchy="false">(</mo> <mi>f</mi> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation> is relatively weakly compact and <InlineEquation ID="IEq10"> <EquationSource Format="TEX">\(f\)</EquationSource> <EquationSource Format="MATHML"><math> <mi>f</mi> </math></EquationSource> </InlineEquation> is said to be strictly uniformly continuous if its double orbit is uniformly equicontinuous on <InlineEquation ID="IEq11"> <EquationSource Format="TEX">\(G\)</EquationSource> <EquationSource Format="MATHML"><math> <mi>G</mi> </math></EquationSource> </InlineEquation>. The <InlineEquation ID="IEq12"> <EquationSource Format="TEX">\(C^{*}\)</EquationSource> <EquationSource Format="MATHML"><math> <mmultiscripts> <mi>C</mi> <mrow /> <mrow> <mrow /> <mo>∗</mo> </mrow> </mmultiscripts> </math></EquationSource> </InlineEquation>-algebras of such functions are denoted, respectively, by <InlineEquation ID="IEq13"> <EquationSource Format="TEX">\(\textit{WS}(G)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi mathvariant="italic">WS</mi> <mo stretchy="false">(</mo> <mi>G</mi> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation> and <InlineEquation ID="IEq14"> <EquationSource Format="TEX">\( UCS (G)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>U</mi> <mi>C</mi> <mi>S</mi> <mo stretchy="false">(</mo> <mi>G</mi> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation>. Then <InlineEquation ID="IEq15"> <EquationSource Format="TEX">\(\textit{WS}(G) \subset UCS (G)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi mathvariant="italic">WS</mi> <mo stretchy="false">(</mo> <mi>G</mi> <mo stretchy="false">)</mo> <mo>⊂</mo> <mi>U</mi> <mi>C</mi> <mi>S</mi> <mo stretchy="false">(</mo> <mi>G</mi> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation> and <InlineEquation ID="IEq16"> <EquationSource Format="TEX">\( AP (G) \subset \textit{WS}(G) \subset WAP (G)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>A</mi> <mi>P</mi> <mo stretchy="false">(</mo> <mi>G</mi> <mo stretchy="false">)</mo> <mo>⊂</mo> <mi mathvariant="italic">WS</mi> <mo stretchy="false">(</mo> <mi>G</mi> <mo stretchy="false">)</mo> <mo>⊂</mo> <mi>W</mi> <mi>A</mi> <mi>P</mi> <mo stretchy="false">(</mo> <mi>G</mi> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation>. <InlineEquation ID="IEq17"> <EquationSource Format="TEX">\(G\)</EquationSource> <EquationSource Format="MATHML"><math> <mi>G</mi> </math></EquationSource> </InlineEquation> is called a <InlineEquation ID="IEq18"> <EquationSource Format="TEX">\( WS \)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi mathvariant="italic">WS</mi> </mrow> </math></EquationSource> </InlineEquation>-group if <InlineEquation ID="IEq19"> <EquationSource Format="TEX">\(\textit{WS}(G) = WAP (G)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi mathvariant="italic">WS</mi> <mo stretchy="false">(</mo> <mi>G</mi> <mo stretchy="false">)</mo> <mo>=</mo> <mi>W</mi> <mi>A</mi> <mi>P</mi> <mo stretchy="false">(</mo> <mi>G</mi> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation>. We will show that if a discrete <i>FC</i>-group <InlineEquation ID="IEq20"> <EquationSource Format="TEX">\(G\)</EquationSource> <EquationSource Format="MATHML"><math> <mi>G</mi> </math></EquationSource> </InlineEquation> is a <InlineEquation ID="IEq21"> <EquationSource Format="TEX">\( WS \)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi mathvariant="italic">WS</mi> </mrow> </math></EquationSource> </InlineEquation>-group, then its center is of finite index in <InlineEquation ID="IEq22"> <EquationSource Format="TEX">\(G\)</EquationSource> <EquationSource Format="MATHML"><math> <mi>G</mi> </math></EquationSource> </InlineEquation>. A noncompact locally compact group <InlineEquation ID="IEq23"> <EquationSource Format="TEX">\(G\)</EquationSource> <EquationSource Format="MATHML"><math> <mi>G</mi> </math></EquationSource> </InlineEquation> is minimally w.a.p., if <InlineEquation ID="IEq24"> <EquationSource Format="TEX">\( WAP (G) = AP (G) \oplus C_{0}(G)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>W</mi> <mi>A</mi> <mi>P</mi> <mrow> <mo stretchy="false">(</mo> <mi>G</mi> <mo stretchy="false">)</mo> </mrow> <mo>=</mo> <mi>A</mi> <mi>P</mi> <mrow> <mo stretchy="false">(</mo> <mi>G</mi> <mo stretchy="false">)</mo> </mrow> <mo>⊕</mo> <msub> <mi>C</mi> <mn>0</mn> </msub> <mrow> <mo stretchy="false">(</mo> <mi>G</mi> <mo stretchy="false">)</mo> </mrow> </mrow> </math></EquationSource> </InlineEquation>. If <InlineEquation ID="IEq25"> <EquationSource Format="TEX">\(G\)</EquationSource> <EquationSource Format="MATHML"><math> <mi>G</mi> </math></EquationSource> </InlineEquation> is minimally w.a.p., then <InlineEquation ID="IEq26"> <EquationSource Format="TEX">\(\textit{WS}(G) = AP (G)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi mathvariant="italic">WS</mi> <mo stretchy="false">(</mo> <mi>G</mi> <mo stretchy="false">)</mo> <mo>=</mo> <mi>A</mi> <mi>P</mi> <mo stretchy="false">(</mo> <mi>G</mi> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation>, i.e., if the double orbit of a bounded continuous function <InlineEquation ID="IEq27"> <EquationSource Format="TEX">\(f\)</EquationSource> <EquationSource Format="MATHML"><math> <mi>f</mi> </math></EquationSource> </InlineEquation> is relatively weakly compact then it is relatively norm compact. It is known that for <InlineEquation ID="IEq28"> <EquationSource Format="TEX">\(n \ge 2\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>n</mi> <mo>≥</mo> <mn>2</mn> </mrow> </math></EquationSource> </InlineEquation>, the motion group <InlineEquation ID="IEq29"> <EquationSource Format="TEX">\(M(n)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>M</mi> <mo stretchy="false">(</mo> <mi>n</mi> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation>, and the special linear group <InlineEquation ID="IEq30"> <EquationSource Format="TEX">\(\textrm{SL}(n,\,\mathbb {R})\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mtext>SL</mtext> <mo stretchy="false">(</mo> <mi>n</mi> <mo>,</mo> <mspace width="0.166667em" /> <mi mathvariant="double-struck">R</mi> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation> are minimally w.a.p. On the other hand, there exist locally compact groups <InlineEquation ID="IEq31"> <EquationSource Format="TEX">\(G\)</EquationSource> <EquationSource Format="MATHML"><math> <mi>G</mi> </math></EquationSource> </InlineEquation> such that <InlineEquation ID="IEq32"> <EquationSource Format="TEX">\(\textit{WS}(G) = AP (G)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi mathvariant="italic">WS</mi> <mo stretchy="false">(</mo> <mi>G</mi> <mo stretchy="false">)</mo> <mo>=</mo> <mi>A</mi> <mi>P</mi> <mo stretchy="false">(</mo> <mi>G</mi> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation> but <InlineEquation ID="IEq33"> <EquationSource Format="TEX">\(G\)</EquationSource> <EquationSource Format="MATHML"><math> <mi>G</mi> </math></EquationSource> </InlineEquation> is not minimally w.a.p. We will show that if <InlineEquation ID="IEq34"> <EquationSource Format="TEX">\(G\)</EquationSource> <EquationSource Format="MATHML"><math> <mi>G</mi> </math></EquationSource> </InlineEquation> is an <i>IN</i>-group and <InlineEquation ID="IEq35"> <EquationSource Format="TEX">\(K = K_{G}\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>K</mi> <mo>=</mo> <msub> <mi>K</mi> <mi>G</mi> </msub> </mrow> </math></EquationSource> </InlineEquation> is the intersection of all closed invariant neighborhoods of the identity of <InlineEquation ID="IEq36"> <EquationSource Format="TEX">\(G\)</EquationSource> <EquationSource Format="MATHML"><math> <mi>G</mi> </math></EquationSource> </InlineEquation>, then <InlineEquation ID="IEq37"> <EquationSource Format="TEX">\( UCS (G) = UCS (G/K)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>U</mi> <mi>C</mi> <mi>S</mi> <mo stretchy="false">(</mo> <mi>G</mi> <mo stretchy="false">)</mo> <mo>=</mo> <mi>U</mi> <mi>C</mi> <mi>S</mi> <mo stretchy="false">(</mo> <mi>G</mi> <mo stretchy="false">/</mo> <mi>K</mi> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation> and <InlineEquation ID="IEq38"> <EquationSource Format="TEX">\(\textit{WS}(G) = \textit{WS}(G/K)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi mathvariant="italic">WS</mi> <mo stretchy="false">(</mo> <mi>G</mi> <mo stretchy="false">)</mo> <mo>=</mo> <mi mathvariant="italic">WS</mi> <mo stretchy="false">(</mo> <mi>G</mi> <mo stretchy="false">/</mo> <mi>K</mi> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation>. We will identify the strictly w.a.p. functions on the <InlineEquation ID="IEq39"> <EquationSource Format="TEX">\(ax + b\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>a</mi> <mi>x</mi> <mo>+</mo> <mi>b</mi> </mrow> </math></EquationSource> </InlineEquation> group. We will also show that <InlineEquation ID="IEq40"> <EquationSource Format="TEX">\( UCS (\textrm{SL}(2,\,\mathbb {R}))\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>U</mi> <mi>C</mi> <mi>S</mi> <mo stretchy="false">(</mo> <mtext>SL</mtext> <mo stretchy="false">(</mo> <mn>2</mn> <mo>,</mo> <mspace width="0.166667em" /> <mi mathvariant="double-struck">R</mi> <mo stretchy="false">)</mo> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation> only contains the constant functions.</p>

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Double orbits of weakly almost periodic functions

  • Ching Chou

摘要

For a locally compact group \(G\) G , let \( AP (G)\) A P ( G ) and \( WAP (G)\) W A P ( G ) be respectively the \(C^{*}\) C -algebras of almost periodic and weakly almost periodic functions on \(G\) G . For a bounded continuous function \(f\) f on \(G\) G , \(f\) f is said to be strictly w.a.p. if its double orbit \(O(f)\) O ( f ) is relatively weakly compact and \(f\) f is said to be strictly uniformly continuous if its double orbit is uniformly equicontinuous on \(G\) G . The \(C^{*}\) C -algebras of such functions are denoted, respectively, by \(\textit{WS}(G)\) WS ( G ) and \( UCS (G)\) U C S ( G ) . Then \(\textit{WS}(G) \subset UCS (G)\) WS ( G ) U C S ( G ) and \( AP (G) \subset \textit{WS}(G) \subset WAP (G)\) A P ( G ) WS ( G ) W A P ( G ) . \(G\) G is called a \( WS \) WS -group if \(\textit{WS}(G) = WAP (G)\) WS ( G ) = W A P ( G ) . We will show that if a discrete FC-group \(G\) G is a \( WS \) WS -group, then its center is of finite index in \(G\) G . A noncompact locally compact group \(G\) G is minimally w.a.p., if \( WAP (G) = AP (G) \oplus C_{0}(G)\) W A P ( G ) = A P ( G ) C 0 ( G ) . If \(G\) G is minimally w.a.p., then \(\textit{WS}(G) = AP (G)\) WS ( G ) = A P ( G ) , i.e., if the double orbit of a bounded continuous function \(f\) f is relatively weakly compact then it is relatively norm compact. It is known that for \(n \ge 2\) n 2 , the motion group \(M(n)\) M ( n ) , and the special linear group \(\textrm{SL}(n,\,\mathbb {R})\) SL ( n , R ) are minimally w.a.p. On the other hand, there exist locally compact groups \(G\) G such that \(\textit{WS}(G) = AP (G)\) WS ( G ) = A P ( G ) but \(G\) G is not minimally w.a.p. We will show that if \(G\) G is an IN-group and \(K = K_{G}\) K = K G is the intersection of all closed invariant neighborhoods of the identity of \(G\) G , then \( UCS (G) = UCS (G/K)\) U C S ( G ) = U C S ( G / K ) and \(\textit{WS}(G) = \textit{WS}(G/K)\) WS ( G ) = WS ( G / K ) . We will identify the strictly w.a.p. functions on the \(ax + b\) a x + b group. We will also show that \( UCS (\textrm{SL}(2,\,\mathbb {R}))\) U C S ( SL ( 2 , R ) ) only contains the constant functions.