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Formal proofs for theoretical properties of Newton's method

Published online by Cambridge University Press:  01 July 2011

IOANA PAŞCA*
Affiliation:
INRIA Sophia Antipolis, 2004 route des Lucioles – BP 93, 06902 Sophia Antipolis Cedex, France Email: Ioana.Pasca@inria.fr
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Abstract

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We discuss a formal development for the certification of Newton's method. We address several issues encountered in the formal study of numerical algorithms: developing the necessary libraries for our proofs, adapting paper proofs to suit the features of a proof assistant and designing new proofs based on the existing ones to deal with optimisations of the method. We start from Kantorovitch's Theorem, which gives the convergence of Newton's method in the case of a system of equations. To formalise this proof inside the proof assistant Coq, we first need to code the necessary concepts from multivariate analysis. We also prove that rounding at each step in Newton's method still yields a convergent process with an accurate correlation between the precision of the input and that of the result. This proof is based on Kantorovitch's Theorem, but is an original result. An algorithm including rounding is a more accurate model for computations with Newton's method in practice.