Fast real and complex root-finding methods for well-conditioned polynomials
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Given a polynomial p of degree d and a bound κ on a condition number of p, we present the first root-finding algorithms that return all its real and complex roots with a number of bit operations quasi-linear in d log^2(κ). More precisely, several condition numbers can be defined depending on the norm chosen on the coefficients of the polynomial. Let p(x) = ∑_k=0^d a_k x^k = ∑_k=0^d √( d k) b_k x^k. We call the condition number associated with a perturbation of the a_k the hyperbolic condition number κ_h, and the one associated with a perturbation of the b_k the elliptic condition number κ_e. For each of these condition numbers, we present algorithms that find the real and the complex roots of p in O(dlog^2(dκ) polylog(log(dκ))) bit operations.Our algorithms are well suited for random polynomials since κ_h (resp. κ_e) is bounded by a polynomial in d with high probability if the a_k (resp. the b_k) are independent, centered Gaussian variables of variance 1.
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