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BIRS Workshop Lecture Videos

Combinatorial and Algebraic Conditions that Preclude SAPpiness Garnett, Colin


It is well known that a complex zero-nonzero pattern cannot be spectrally arbitrary if its digraph doesn’t have at least two loops and at least one two cycle, or at least three loops. This talk focuses on several other combinatorial conditions on the digraph that preclude it from being spectrally arbitrary. In particular we are sometimes able to reduce the number of unknown entries to be below the threshold of $2n−1$. Furthermore there are several algebraic conditions on the coefficients of the characteristic polynomial that can be exploited to show that a pattern is not spectrally arbitrary over any field. Using Sage we were able to show that no zero-nonzero pattern with $2n−1$ nonzero entries will be spectrally arbitrary over $C$ where $n≤6$. When $n=7$ we find two zero-nonzero patterns that do not satisfy our algebraic conditions precluding them from being spectrally arbitrary.

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