I am reading the proof of the Choi's theorem in his own paper.(adsbygoogle = window.adsbygoogle || []).push({});

he first introduces ##E_{ij} ## as the nn null matrix but with a 1 at i,j.

Then he uses ##(E_{ij})_{1<=i,j <=n}##

he says that thi is a positive matrix. What is talking about?

Is it a matrix of matrices?

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# B What is this notation?

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