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Staring with an equilateral triangle $\Delta$, inscribe a circle, then in the gaps, inscribe other circles, ad infinitum. Similarly, inside the circumscribed circle but outside $\Delta$, continue to fill in every gap with a touching circle:


      InscribedCircumscribed


Q. Is some inscribed radius equal to some circumscribed radius, among all the gap-filling circles? Is ever a blue circle congruent to a red circle? Or do they each generate distinct infinite series of radii that never share a radius?

I would especially appreciate a resolution without resorting to explicit calculation of every radius.

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    If you look at the primary "chain" of circles (i.e. touching the perimeter of the segment or triangle),the sequences of positions and radii are recurrent. There are similarities with https://math.stackexchange.com/questions/3591844/chain-of-circles-internally-tangent-to-an-ellipse/3640977#3640977, a question a chain of circles in an ellipse. There the answer is less about geometry and more about the properties of Lucas sequences. I speculate that it is a similar story here, albeit more complicated because we're dealing with more than just the primary chains. – brainjam Dec 14 '20 at 03:28

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