# Fibonacci word fractal

The Fibonacci word fractal is a fractal curve defined on the plane from the Fibonacci word.

## Definition

The first iterations

L-system representation[1]

This curve is built iteratively by applying, to the Fibonacci word 0100101001001...etc., the Odd–Even Drawing rule:

For each digit at position k :

1. Draw a segment forward
2. If the digit is 0:
• Turn 90° to the left if k is even
• Turn 90° to the right if k is odd

To a Fibonacci word of length ${\displaystyle F_{n}}$  (the nth Fibonacci number) is associated a curve ${\displaystyle {\mathcal {F}}_{n}}$  made of ${\displaystyle F_{n}}$  segments. The curve displays three different aspects whether n is in the form 3k, 3k + 1, or 3k + 2.

## Properties

The Fibonacci numbers in the Fibonacci word fractal.

Some of the Fibonacci word fractal's properties include:[2][3]

• The curve ${\displaystyle {\mathcal {F_{n}}}}$ , contains ${\displaystyle F_{n}}$  segments, ${\displaystyle F_{n-1}}$  right angles and ${\displaystyle F_{n-2}}$  flat angles.
• The curve never self-intersects and does not contain double points. At the limit, it contains an infinity of points asymptotically close.
• The curve presents self-similarities at all scales. The reduction ratio is ${\displaystyle \scriptstyle {1+{\sqrt {2}}}}$ . This number, also called the silver ratio is present in a great number of properties listed below.
• The number of self-similarities at level n is a Fibonacci number \ −1. (more precisely : ${\displaystyle F_{3n+3}-1}$ ).
• The curve encloses an infinity of square structures of decreasing sizes in a ratio ${\displaystyle \scriptstyle {1+{\sqrt {2}}}}$ . (see figure) The number of those square structures is a Fibonacci number.
• The curve ${\displaystyle {\mathcal {F}}_{n}}$ can also be constructed by different ways (see gallery below):
• Iterated function system of 4 and 1 homothety of ratio ${\displaystyle \scriptstyle {1/(1+{\sqrt {2}})}}$  and ${\displaystyle \scriptstyle {1/(1+{\sqrt {2}})^{2}}}$
• By joining together the curves ${\displaystyle {\mathcal {F}}_{n-1}}$  and ${\displaystyle {\mathcal {F}}_{n-2}}$
• Lindenmayer system
• By an iterated construction of 8 square patterns around each square pattern.
• By an iterated construction of octagons
• The Hausdorff dimension of the Fibonacci word fractal is ${\displaystyle \scriptstyle {3{\frac {\log \varphi }{\log(1+{\sqrt {2}})}}\approx 1.6379}}$ , with ${\displaystyle \scriptstyle {\varphi ={\frac {1+{\sqrt {5}}}{2}}}}$ , the golden ratio.
• Generalizing to an angle ${\displaystyle \alpha }$  between 0 and ${\displaystyle \pi /2}$ , its Hausdorff dimension is ${\displaystyle \scriptstyle {3{\frac {\log \varphi }{\log(1+a+{\sqrt {(1+a)^{2}+1}})}}}}$ , with ${\displaystyle a=\cos \alpha }$ .
• The Hausdorff dimension of its frontier is ${\displaystyle \scriptstyle {{\frac {\log 3}{{\log(1+{\sqrt {2}}})}}\approx 1.2465}}$ .
• Exchanging the roles of "0" and "1" in the Fibonacci word, or in the drawing rule yields a similar curve, but oriented 45°.
• From the Fibonacci word, one can define the « dense Fibonacci word», on an alphabet of 3 letters : 102210221102110211022102211021102110221022102211021... ((sequence A143667 in the OEIS)). The usage, on this word, of a more simple drawing rule, defines an infinite set of variants of the curve, among which :
• a "diagonal variant"
• a "svastika variant"
• a "compact variant"
• It is conjectured that the Fibonacci word fractal appears for every sturmian word for which the slope, written in continued fraction expansion, ends with an infinite series of "1".

## The Fibonacci tile

Imperfect tiling by the Fibonacci tile. The area of the central square tends to infinity.

The juxtaposition of four ${\displaystyle F_{3k}}$  curves allows the construction of a closed curve enclosing a surface whose area is not null. This curve is called a "Fibonacci Tile".

• The Fibonacci tile almost tiles the plane. The juxtaposition of 4 tiles (see illustration) leaves at the center a free square whose area tends to zero as k tends to infinity. At the limit, the infinite Fibonacci tile tiles the plane.
• If the tile is enclosed un{Clarification} a square of side 1, then its area tends to ${\displaystyle \scriptstyle {2-{\sqrt {2}}=0.5857}}$ .

Perfect tiling by the Fibonacci snowflake

### Fibonacci snowflake

Fibonacci snowflakes for i=2 for n=1 through 4: ${\displaystyle \sideset {}{_{1}^{\left[2\right]}\quad }\prod }$ , ${\displaystyle \sideset {}{_{2}^{\left[2\right]}\quad }\prod }$ , ${\displaystyle \sideset {}{_{3}^{\left[2\right]}\quad }\prod }$ , ${\displaystyle \sideset {}{_{4}^{\left[2\right]}\quad }\prod }$ [4]

The Fibonacci snowflake is a Fibonacci tile defined by:[5]

• ${\displaystyle \scriptstyle {q_{n}=q_{n-1}q_{n-2}}}$  if ${\displaystyle \scriptstyle {n\equiv 2{\pmod {3}}}}$
• ${\displaystyle \scriptstyle {q_{n}=q_{n-1}{\overline {q}}_{n-2}}}$  otherwise.

with ${\displaystyle q_{0}=\epsilon }$  and ${\displaystyle q_{1}=R}$ , ${\displaystyle L=}$ "turn left" et ${\displaystyle R=}$ "turn right", and ${\displaystyle \scriptstyle {{\overline {R}}=L}}$ ,

Several remarkable properties :[5] · :[6]

• It is the Fibonacci tile associated to the "diagonal variant" previously defined.
• It tiles the plane at any order.
• It tiles the plane by translation in two different ways.
• its perimeter, at order n, equals ${\displaystyle 4F(3n+1)}$ . ${\displaystyle F(n)}$  is the nth Fibonacci number.
• its area, at order n, follows the successive indexes of odd row of the Pell sequence (defined by ${\displaystyle P(n)=2P(n-1)+P(n-2)}$ ).