show/hide this revision's text 2 answered the new question in the comment

Yes, of course, after we shrink $U$. A convergent Taylor series at a real point converges in some complex neighborhood of this point.

Added reply to your comment: you can apply identity theorem. Two real analytic functions coinciding on an open set of $R^n$ coincide in a complex neighborhood of this set.

show/hide this revision's text 1

Yes, of course, after we shrink $U$. A convergent Taylor series at a real point converges in some complex neighborhood of this point.