The paving conjecture for zero-diagonal operators

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Let ell2nell_2^n be the nn-dimensional Hilbert space, let TT be a linear operator on ell2nell_2^n whose matrix has zero diagonal, and for A◃{1,…,n}A\triangleleft\{1,\ldots,n\} let QAQ_A denote the orthogonal projection onto the coordinates indexed by AA. A partition cA={Aj}j=1rcA=\{A_j\}_{j=1}^r of cbrace{1,\ldots,n\} is called a paving of TT if it has rr parts. The paving conjecture. For every ϵ>0\epsilon>0, there exists a natural number rr such that, for every natural number nn and every such operator TT, there is a paving cA={Aj}j=1rcA=\{A_j\}_{j=1}^r satisfying

∥QAjTQAj∥\leqepsilon∥T∥for all j=1,…,r.\|Q_{A_j}TQ_{A_j}\|\leqepsilon\|T\|\qquad\text{for all }j=1,\ldots,r.

This conjecture is equivalent to the Kadison–Singer problem and concerns uniform blockwise smallness of zero-diagonal operators. The supplied source does not establish its resolution, so its status is recorded as open.

References

Primary source

Peter G. Casazza, Matt Fickus, Dustin Mixon and Janet C. Tremain, “Concrete constructions of non-pavable projections”, arXiv:1005.2164 (2010).

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