Coxeter's loxodromic sequence of tangent circles

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Blue circle 0 is tangent to circles 1, 2 and 3, as well as to preceding circles −1, −2 and −3.
Blue circle 0 is tangent to circles 1, 2 and 3, as well as to preceding circles −1, −2 and −3.

In geometry, Coxeter's loxodromic sequence of tangent circles is an infinite sequence of circles arranged such that any four consecutive circles in the sequence are pairwise mutually tangent. This means that each circle in the sequence is tangent to the three circles that precede it and also to the three circles that follow it.

The radii of the circles in the sequence form a geometric progression with ratio

k=\varphi + \sqrt{\varphi} \approx 2.89005 \ ,

where φ is the golden ratio. k and its reciprocal satisfy the equation

(1+x+x^2+x^3)^2=2(1+x^2+x^4+x^6)\ ,

and so any four consecutive circles in the sequence meet the conditions of Descartes' theorem.

The centres of the circles in the sequence lie on a logarithmic spiral. Viewed from the centre of the spiral, the angle between the centres of successive circles is

\cos^{-1} \left( \frac {-1} {\varphi} \right) \approx 128.173 ^ \circ \ .

The construction is named after geometer Donald Coxeter, who generalised the two-dimensional case to sequences of spheres and hyperspheres in higher dimensions.

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