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While the eigenvalues of a diagonal matrix are, of course, equal to its diagonal entries, Gershgorin’s circle theorem (Gershgorin 31, Prop. below) provides upper bounds (the Gershgorin radii, Def. below) for general square matrices over the complex numbers on how far, in the complex plane, the eigenvalues can be from the values of the diagonal entries.
Let be a natural number.
(Gershgorin discs)
Let be a square matrix with complex entries. For define
the th Gershgorin radius of to be the sum of absolute values of off-diagonal entries in the th row:
the th Gershgorin disc to be the closed disk in the complex plane centered at the th diagonal entry with radius the above th Gershgorin radius:
(Gershgorin disc theorem)
For a square matrix with complex entries, each of its eigenvalues is contained in at least one Gershgorin disc (Def. ):
(see, e.g., Meckes 19, Thm. 7.1)
In the context of Prop. , if an eigenvalue has mulitplicity , then it is contained in at least Gershgorin discs (Def. ).
This is due to Marsli-Hall 13.
If the matrix has non-negative values and an eigenvalue has multiplicity greater than 1, then it is contained within this disc of half the radius inside one of the Gershgorin discs (2).
(Barany-Solymosi 16, Theorem 1)
The original article:
Textbook account:
Lecture notes:
Exposition:
See also:
Strengthening for eigenvalues with higher multiplicity:
Further strengthening for matrices with, in addition, non-negative entries:
Last revised on November 19, 2023 at 00:36:03. See the history of this page for a list of all contributions to it.