Polynomial root clustering and explicit deflation

Abstract: We seek complex roots of a univariate polynomial $P$ with real or complex coefficients. We address this problem based on recent algorithms that use subdivision and have a nearly optimal complexity. They are particularly efficient when only roots in a given Region Of Interest (ROI) are sought. We propose two improvements for root finders. The first one is applied to polynomials having only real coefficients; their roots are either real or appear in complex conjugate pairs. We show how to adapt the subdivision scheme to focus the computational effort on the imaginary positive part of the ROI. In our second improvement we deflate $P$ to decrease its degree and the arithmetic cost of the subdivision.