, Volume 16, Issue 1, pp 53–66 | Cite as

The Kadison–Singer problem for the direct sum of matrix algebras

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Let Mn denote the algebra of complex n × n matrices and write M for the direct sum of the Mn. So a typical element of M has the form
$$ x = x_1\oplus x_2 \cdots \oplus x_n \oplus \cdots, $$
where \({x_n \in M_n}\) and \({\|x\| = \sup_n\|x_n\|}\). We set \({D= \{\{x_n\}\in M: x_n\,{\rm is\,diagonal\,for\,all}\,N\}}\). We conjecture (contra Kadison and Singer in Am J Math 81:383–400, 1959) that every pure state of D extends uniquely to a pure state of M. This is known for the normal pure states of D, and we show that this is true for a (weak*) open, dense subset of all the singular pure states of D. We also show that (assuming the Continuum hypothesis) M has pure states that are not multiplicative on any maximal abelian *-subalgebra of M.


Kadison–Singer problem Matrix algebras Operator algebras 

Mathematics Subject Classification (2000)

Primary 46L 


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© The Author(s) 2011

Authors and Affiliations

  1. 1.Department of MathematicsUniversity of CaliforniaSanta BarbaraUSA
  2. 2.Department of MathematicsPenn State UniversityUniversity ParkUSA
  3. 3.Department of MathematicsBogazici UniversityBebek, IstanbulTurkey

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