- #1
Pere Callahan
- 586
- 1
Hi,
I was wondering whether two rational functions f,g whch coincide on the unit circle actually coincide on all of C.
I would say yes. Let D be the set of all complex numbers with the poles of both f and g removed (let's assume there are no poles on the unit circle). This is then open and connected, hence a domain and f and g are analytic there. Moreover they agree on the unit circle which is a set with at least one nonisolated point (in fact all points are nonisolated) and which lies in D, so the uniqueness principle implies that f and g agree on D.
But the poles have to be the same as well. For if w is a pole of f but not of g then the limit of f as z approaches w is infinity and must be the same as the limit of g as w approaches infinty, because a neighbourhood of z is contained in D.
Is this correct?
thanks
I was wondering whether two rational functions f,g whch coincide on the unit circle actually coincide on all of C.
I would say yes. Let D be the set of all complex numbers with the poles of both f and g removed (let's assume there are no poles on the unit circle). This is then open and connected, hence a domain and f and g are analytic there. Moreover they agree on the unit circle which is a set with at least one nonisolated point (in fact all points are nonisolated) and which lies in D, so the uniqueness principle implies that f and g agree on D.
But the poles have to be the same as well. For if w is a pole of f but not of g then the limit of f as z approaches w is infinity and must be the same as the limit of g as w approaches infinty, because a neighbourhood of z is contained in D.
Is this correct?
thanks