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Paper   IPM / M / 18400
School of Mathematics
  Title:   Minimum trace norm of real symmetric and Hermitian matrices with zero diagonal
  Author(s):  Mostafa Einollahzadeh (Joint with M.A. Nematollahi)
  Status:   Published
  Journal: Special Matrices
  Vol.:  13
  Year:  2025
  Pages:   1-16
  Supported by:  IPM
  Abstract:
We establish tight lower bounds for the trace norm \((\|{\cdot}\|_1)\) of real symmetric and Hermitian matrices with zero diagonal entries in terms of their entrywise \(L^1\)-norms \((\|{\cdot}\|_{(1)})\). For the space of nonzero real symmetric matrices of order \(n\), we prove that the minimum possible ratio \(\frac{\|A\|_1}{\|A\|_{(1)}}\) is exactly \(\frac{2}{n}\). In the Hermitian case, this minimum ratio is given by \(\tan\left(\frac{\pi}{2n}\right)\).

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