# Curves and L-systems – by Richard Inexperienced

*by*Phil Tadros

The image above exhibits an approximation to the *Gosper curve*, which was found by **Invoice Gosper** in 1973. The Gosper curve is a *space-filling curve*, which signifies that it converges to a curve that utterly fills within the inside of a form. The form in query (proven beneath) is known as the *Gosper island*. Regardless of having a fractal boundary, the Gosper island has a property referred to as “rep-7”, which signifies that it may be break up up into 7 equivalent smaller copies of itself. This provides a strategy to tile the aircraft by utilizing copies of the Gosper island.

Shapes which have the property rep-*n* for some worth of *n* are collectively often called *rep-tiles*. There are various examples of rep-tiles, equivalent to these proven within the image beneath. You could prefer to see if you’ll find the 2 “fish”, and guess which of the shapes is known as the “sphinx”.

Fractals just like the Gosper curve can usually be constructed utilizing *L-systems*. The “L” stands for Lindenmayer, after the theoretical biologist **Aristid Lindenmayer**, who launched them in 1968. An L-system consists of an *alphabet*, an *preliminary state*, and a set of *manufacturing guidelines*. Lindenmayer’s unique system was designed to mannequin the expansion of algae. It has an alphabet of two letters, {A, B}, an preliminary state of A, and two manufacturing guidelines: (A ↦ AB) and (B ↦ A). The manufacturing guidelines point out that at every iteration of the process, we substitute every prevalence of “A” by “AB”, and every prevalence of “B” by “A”. Iterating this process, we get hold of the sequence A, AB, ABA, ABAAB, ABAABABA, ABAABABAABAAB, and so forth. This sequence converges to an infinitely lengthy phrase referred to as the *Fibonacci phrase*. If we take any of the phrases on this sequence, for instance ABAABABAABAAB, we discover that the variety of As (eight on this case), the variety of Bs (5), and the overall variety of letters (13) are all *Fibonacci numbers*. (The Fibonacci numbers 1, 1, 2, 3, 5, 8, 13, … have the property that every quantity past the primary two is the sum of the earlier two numbers.)

Different L-systems can be utilized to mannequin the expansion of vegetation, and to generate pretty life like photos, like those above.

The L-system for the Gosper curve has an alphabet consisting of 4 symbols: {A, B, +, –}. The preliminary state is A, and there are two manufacturing guidelines: (A ↦ A – B – – B + A + + A A + B –) and (B ↦ + A – B B – – B – A + + A + B). The ensuing strings ought to be considered directions to a graphics turtle that’s drawing the Gosper curve: the letters “A” and “B” every imply “transfer ahead”, “+” means “flip left 60°”, and “–” means “flip proper 60°”.

The *terdragon curve* is one other curve that may be constructed with an L-system. (The curve is a triangular model of one other curve referred to as the *dragon curve*, which is presumably the place the identify comes from.) The terdragon curve covers the aircraft within the sense that it will definitely visits each edge in a triangular lattice. The L-system on this case has an alphabet of three symbols, {F, +, –}. The preliminary state is F, and there’s a single manufacturing rule: (F ↦ F – F + F). The image “F” means “transfer ahead”, “–” means “flip proper 120°”, and “+” means “flip left 120°”. The primary six steps of the iteration are proven within the image above. The image beneath is a 3D sculpture displaying a creating terdragon curve.

I discovered concerning the terdragon curve from the current paper *Coverings of the plane by self-avoiding curves which satisfy the local isomorphism property *by **Francis Oger**. Oger considers a generalization of the terdragon curve during which there are two variations of the manufacturing rule, (F ↦ F + F – F) and (F ↦ F – F + F), and these could be utilized in an arbitrary order. The *kind* of such a curve is the (infinite) sequence of manufacturing guidelines used to assemble it. Oger proves that totally different sequences of manufacturing guidelines give rise to totally different curves, in a way made exact within the paper, and that if Λ is any sequence of manufacturing guidelines, then it’s doable to cowl the aircraft in a singular manner with copies of curves of kind Λ.

**Image credit and related hyperlinks**

The image of the terdragon curve comes from the paper *Growing fractal curves* by **Geoffrey Irving** and **Henry Segerman**. The 3D sculpture proven is by **Henry Segerman** and was exhibited within the SFO Museum. The paper by Irving and Segerman accommodates many different putting photos.

The image of the Gosper curve is by **Arbol01** and **Blotwell**. The photographs of the Gosper island are by **Inductiveload. **All these photos seem on Wikipedia.

The rep-tile image is by **MagistraMundi** and seems on Wikipedia.

The substitute plant photos are by **Solkoll** and seem on the Wikipedia web page on L-systems.

Wikipedia has a web page concerning the Fibonacci word.

**Jeffrey Ventrella**’s YouTube video Growing the Ter-Dragon exhibits the primary few steps of the terdragon curve as an animation.