<p>Inspired by the classical Bohman-Korovkin-Wulbert (BKW) operators, we initiate a study of noncommutative BKW-operators. Let <i>A</i> be a unital <InlineEquation ID="IEq1"> <EquationSource Format="TEX">\(C^{*}\)</EquationSource> <EquationSource Format="MATHML"><math> <mmultiscripts> <mi>C</mi> <mrow /> <mrow> <mrow /> <mo>∗</mo> </mrow> </mmultiscripts> </math></EquationSource> </InlineEquation>-algebra, and <i>S</i> be a set of generators of <i>A</i>. A unital completely positive (UCP)-map <InlineEquation ID="IEq2"> <EquationSource Format="TEX">\(\phi : A\rightarrow B(H)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>ϕ</mi> <mo>:</mo> <mi>A</mi> <mo stretchy="false">→</mo> <mi>B</mi> <mo stretchy="false">(</mo> <mi>H</mi> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation> is said to be a <i>noncommutative BKW-operator</i> for <i>S</i> with respect to norm or weak operator topology (WOT) or strong operator topology (SOT) if for any sequence of UCP-maps <InlineEquation ID="IEq3"> <EquationSource Format="TEX">\(\phi _n:A\rightarrow B(H)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msub> <mi>ϕ</mi> <mi>n</mi> </msub> <mo>:</mo> <mi>A</mi> <mo stretchy="false">→</mo> <mi>B</mi> <mrow> <mo stretchy="false">(</mo> <mi>H</mi> <mo stretchy="false">)</mo> </mrow> </mrow> </math></EquationSource> </InlineEquation>, <InlineEquation ID="IEq4"> <EquationSource Format="TEX">\(n=1,2,...,\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>n</mi> <mo>=</mo> <mn>1</mn> <mo>,</mo> <mn>2</mn> <mo>,</mo> <mo>.</mo> <mo>.</mo> <mo>.</mo> <mo>,</mo> </mrow> </math></EquationSource> </InlineEquation> <InlineEquation ID="IEq5"> <EquationSource Format="TEX">\(\lim _{n\rightarrow \infty }\phi _n(s)=\phi (s),\forall ~s\in S\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msub> <mo movablelimits="true">lim</mo> <mrow> <mi>n</mi> <mo stretchy="false">→</mo> <mi>∞</mi> </mrow> </msub> <msub> <mi>ϕ</mi> <mi>n</mi> </msub> <mrow> <mo stretchy="false">(</mo> <mi>s</mi> <mo stretchy="false">)</mo> </mrow> <mo>=</mo> <mi>ϕ</mi> <mrow> <mo stretchy="false">(</mo> <mi>s</mi> <mo stretchy="false">)</mo> </mrow> <mo>,</mo> <mo>∀</mo> <mspace width="3.33333pt" /> <mi>s</mi> <mo>∈</mo> <mi>S</mi> </mrow> </math></EquationSource> </InlineEquation> in norm (or WOT or SOT) <InlineEquation ID="IEq6"> <EquationSource Format="TEX">\(\Rightarrow \lim _{n\rightarrow \infty }\phi _n(a)=\phi (a), \forall ~a\in A\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mo stretchy="false">⇒</mo> <msub> <mo movablelimits="true">lim</mo> <mrow> <mi>n</mi> <mo stretchy="false">→</mo> <mi>∞</mi> </mrow> </msub> <msub> <mi>ϕ</mi> <mi>n</mi> </msub> <mrow> <mo stretchy="false">(</mo> <mi>a</mi> <mo stretchy="false">)</mo> </mrow> <mo>=</mo> <mi>ϕ</mi> <mrow> <mo stretchy="false">(</mo> <mi>a</mi> <mo stretchy="false">)</mo> </mrow> <mo>,</mo> <mo>∀</mo> <mspace width="3.33333pt" /> <mi>a</mi> <mo>∈</mo> <mi>A</mi> </mrow> </math></EquationSource> </InlineEquation> in norm (or WOT or SOT, respectively). We identify a connection between noncommutative BKW-operators and the unique CP-extension of UCP-maps. We have discussed several examples and explored different notions of noncommutative BKW-operators and their interconnections. Additionally, we introduce the concept of hyperrigidity with respect to a UCP-map and characterize it along the lines of Arveson. Although independent yet related to noncommutative BKW-operators, we provide a noncommutative version of the operator version of the Korovkin theorem recently proposed by D. Popa.</p>

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Noncommutative BKW-operators

  • C. S. Arunkumar,
  • Sruthymurali

摘要

Inspired by the classical Bohman-Korovkin-Wulbert (BKW) operators, we initiate a study of noncommutative BKW-operators. Let A be a unital \(C^{*}\) C -algebra, and S be a set of generators of A. A unital completely positive (UCP)-map \(\phi : A\rightarrow B(H)\) ϕ : A B ( H ) is said to be a noncommutative BKW-operator for S with respect to norm or weak operator topology (WOT) or strong operator topology (SOT) if for any sequence of UCP-maps \(\phi _n:A\rightarrow B(H)\) ϕ n : A B ( H ) , \(n=1,2,...,\) n = 1 , 2 , . . . , \(\lim _{n\rightarrow \infty }\phi _n(s)=\phi (s),\forall ~s\in S\) lim n ϕ n ( s ) = ϕ ( s ) , s S in norm (or WOT or SOT) \(\Rightarrow \lim _{n\rightarrow \infty }\phi _n(a)=\phi (a), \forall ~a\in A\) lim n ϕ n ( a ) = ϕ ( a ) , a A in norm (or WOT or SOT, respectively). We identify a connection between noncommutative BKW-operators and the unique CP-extension of UCP-maps. We have discussed several examples and explored different notions of noncommutative BKW-operators and their interconnections. Additionally, we introduce the concept of hyperrigidity with respect to a UCP-map and characterize it along the lines of Arveson. Although independent yet related to noncommutative BKW-operators, we provide a noncommutative version of the operator version of the Korovkin theorem recently proposed by D. Popa.