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If we have two sequences of degree $n$ monic polynomials $f_n(z)$ and $g_n(z)$, with zeros of $f_n, g_n$ lies outside the unit circle, i.e. in $\{z:|z|\ge 1\}$, and for any $r\in(0,1)$ we know $\lim_{n\to\infty}\sup_{|z|\le r}|f_n(z)-g_n(z)|=0$. What can we say about the zeros of $f_n$ and $g_n$? Is there any properties they should share in common?

I know the support and the counting measures of these two zeros sets can be very different, in fact it's possible that one lies on the unit circle and another one does not. But for example, is it necessarily true that their angular projections onto the unit circle are close to each other?

Or is there any other common properties that can be derived from that closeness of two polynomials inside the circle?

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  • $\begingroup$ If you find it helpful, you can also assume we know the convergent rate to zero. Feel free to add anything necessary to conclude a meaningful statement. $\endgroup$ Commented Jan 27 at 11:24
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    $\begingroup$ You must normalize the coefficients somehow (for example consider monic polynomials), otherwise you can just take any two polynomials and multiply them by small constants. $\endgroup$ Commented Jan 27 at 15:26
  • $\begingroup$ @ChristianRemling You are absolutely right. Previous examples in my mind are (inverse) characteristic polynomials of matrices, so indeed monic if we require det=1. I've added conditions accordingly. $\endgroup$ Commented Jan 27 at 16:38
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    $\begingroup$ Convergence inside the disk tells basically nothing about what happens outside. For example, consider $f_n(z)=z^n+2$ and $g_n(z)=z^n+A_nz^{n-1}+2$. These polynomials satisfy your conditions if $A_n$ grows slower than exponential, but the zeros (or the arguments) don't have to be closed. $\endgroup$ Commented Jan 27 at 17:58

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