Circularity of the numerical range. (English) Zbl 0809.15012
The authors give an equivalent condition on a 3 or 4-square real upper triangular matrix for its numerical range to be a circular disk centered at the origin. Some sufficient condition on certain sparse matrices for the same problem are also obtained.
Reviewer: Li Shaokuan (Shanghai)
MSC:
| 15A60 | Norms of matrices, numerical range, applications of functional analysis to matrix theory |
References:
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