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Evaluating winding numbers and counting complex roots through Cauchy indices in Isabelle/HOL. (English) Zbl 07176600
Summary: In complex analysis, the winding number measures the number of times a path (counter-clockwise) winds around a point, while the Cauchy index can approximate how the path winds. We formalise this approximation in the Isabelle theorem prover, and provide a tactic to evaluate winding numbers through Cauchy indices. By further combining this approximation with the argument principle, we are able to make use of remainder sequences to effectively count the number of complex roots of a polynomial within some domains, such as a rectangular box and a half-plane.
##### MSC:
 68V15 Theorem proving (automated and interactive theorem provers, deduction, resolution, etc.)
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##### References:
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