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Rauzy Fractal


The Rauzy fractal is a planar fractal associated with the tribonacci constant and the substitution system 0->01, 1->02, 2->0 (Rauzy 1982, pp. 148-150; Messaoudi 1998, p. 136). It connects numeral systems, tilings, and dynamical systems.

Let alpha be the nonreal root of

 x^3-x^2-x-1=0
(1)

with positive imaginary part. The real root is the tribonacci constant beta>1, and |alpha|=beta^(-1/2)<1 (Rauzy 1982, p. 150). One normalization of the Rauzy fractal is

 K={sum_(k=0)^inftyepsilon_kalpha^k:epsilon_k in {0,1}, epsilon_kepsilon_(k+1)epsilon_(k+2)=0 for all k>=0}.
(2)

Thus the binary digits may not contain three consecutive 1s. Messaoudi (1998, p. 136) uses the scaled normalization E=alpha^3K.

RauzyFractalPieces

The three pieces are illustrated above using finite sums. Grouping the admissible digit strings by their initial blocks 0, 10, and 110 gives the three affine pieces

 K=alphaK union (1+alpha^2K) union (1+alpha+alpha^3K).
(3)

The corresponding contraction mappings form an iterated function system.

RauzyFractalTiling

The scaled set E=alpha^3K tiles the complex plane by translations from the point lattice Z+Zalpha (Messaoudi 1998, p. 138). The illustration shows finite approximations of E and its six neighboring translates by +/-1, +/-alpha, and +/-(1+alpha). Its three pieces also give a nonperiodic tiling (Messaoudi 1998, p. 136).

The boundary of K is a Jordan curve (Messaoudi 2000, p. 208). Its Hausdorff dimension is

 dim_H(partialK)=(lnrho)/(ln|alpha|)=1.0933641...,
(4)

where rho is the largest real root of x^4+2x^3-1=0 (Messaoudi 2000, pp. 210-211). The scaling between K and E does not change this Hausdorff dimension.

Related Rauzy fractal constructions arise from other substitution systems. For example, A->AC, B->AB, C->B has dominant eigenvalue P^2, where P is the plastic constant, and gives three subtiles (Coulbois and Minervino 2017, pp. 6 and 12-13). The limiting fractal in the pentagonal construction for the plastic constant is similar to one of these subtiles in complex eigenvector coordinates.

RauzyFractalGjiniComparison

Another example is the central tile studied by Gjini (2001, pp. 305-306), associated with the supergolden ratio and digit strings in which each 1 is followed by at least two 0s. The figure above compares finite approximations of this central tile at left and the tent tile for the same parameter at right, scaled independently for display. Writing a for the root of x^3-x^2-1=0 with positive imaginary part, the central tile satisfies K=aK union (1+a^3K) (Gjini 2001, p. 306), while the tent tile satisfies F=aF union a^3(1-F) (Scheicher et al. 2026). The boundary of the central tile has Hausdorff dimension approximately 1.47131 (Gjini 2001, pp. 317-319). The boundary of the tent tile has Hausdorff dimension approximately 1.02952 (Scheicher et al. 2026), so the two attractors are not similar. This illustrates why an underlying algebraic number alone does not determine a Rauzy fractal.


See also

Fractal, Hokkaido Fractal, Iterated Function System, Nonperiodic Tiling, Numeral System, Pisot Number, Substitution System, Tent Tile, Tribonacci Constant, Tribonacci Sequence

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References

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Cite this as:

Weisstein, Eric W. "Rauzy Fractal." From MathWorld--A Wolfram Resource. https://mathworld.wolfram.com/RauzyFractal.html

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