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Meromorphic Extendibility and the Argument Principle

Josip Globevnik

math.CVarXiv:math/0612031

Abstract

Let U be the open unit disc in C. Given a continuous function g: bU --> C-0 denote by W(g) the winding number of g around the origin. We prove that a continuous function f: bU --> C extends meromorphically through U if and only if there is a nonnegative integer N such that W(Pf+Q) is greater than or equal to -N for every pair P,Q of polynomials such that Pf+Q has no zero on bU. If this is the case then the meromorphic extension of f has at most N poles in U, counting multiplicity.

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