Dragon Evolution



How to derive a V-Dragon from a V-Tile

We start the dragon evolution with a V-Seahorse and not with a V-polygon. In this way we avoid changes of the neck vertices after several single modifications. We need a seahorse with wide head, otherwise intersections would occur in step 04.
Here we are not forced to use an angle ε = 180°/k, because the resulting V-Dragon will not admit spiral or rotational tilings of the plane. Suitable angles are from the interval [12°,18°], but in principle any angle greater than 0° can be used if it is small enough that no intersections of the circular arcs occur (this depends on the position of point A1).


Step 01

For our sample we use a V-Seahorse with wide head and internal angle ε = 15° at point Z. A good choice for the internal angle at point T is 180°-2ε, because then the angle at V is equal to ε.
Keep the point A1 on the left ray from T variable. This allows changes the size of the head and the thickness of the neck.



Step 02

Construct V-Seahorse 2 and 3 by two transformations of the original tile 1. Namely refelction of tile 1 at point S and rotation of tile 1 around Z by ε. Tile 1 is incompletely surrounded by tile 2 and 3.


Our goal is to construct a patch topologically equivant to a disc and formed by three congruent tiles, where tile 1 is surrounded by the tiles 2 and 3 Additionally 3 conditions have to be met:

Conditions

(1) Instead of only point V, tile 2 and 3 have a line segment in common near the bottom of the patch.
(2) Instead of only point Z, tile 2 and 3 have a line segment in common near the top of the patch.
(3) Couples of tile 1 and 2 admit a tiling of the plane.



Step 03

First we consider point V that belongs to all three tiles. Correctly this is point V of tile 1, point Z of tile 2 and point W of tile 3. By adding the green rectangle to tile 3 we achieve that tile 2 and 3 have a line segment in common. The point V1 does not belong to the boundary of patch any more. Let F3 be the upper left point of the rectangle. The position of F3 on V3V1 shall be variable.



Step 04

As the three tiles have to be congruent, we need to add the rectangle also to the 'feet' of tile 1 and 2. As the tile 2 is upside down its foot is near the top of the patch.



Step 05

The rectangle added to tile 1 cuts off a rectangle in the head of tile 2 and vice versa. This cut also has to occur in the head of tile 3. We obtain F3' by reflecting F3 at point R. The new rectangle with F3' as lower left corner produces a hole in the patch. We have to fill this hole by adding a rectangle to the head of tile 1. In the following step we write H1 instead of F3' because this point belongs to the head of tile 1.



Step 06

The rectangle added to the head of tile 1 also has to be added to the heads of tile 3 (top of the patch) and tile 2 (bottom of the patch). We obtain H2 by reflection of H1 at point S and H3 by rotation of H1 around Z by ε.
Now all tiles are congruent and the patch meets condition 1.
Normally there is a gap between the two rectangles on the bottom of the patch.



Step 07

In general the gap described in step 06 prevents a tiling of the plane by the tile couple 1 and 2. To resolve this problem we can close the gap by using a length for the rectangle that is equal to the length of TZ. This may lead to overlapings of the tiles. Then we have to adjust F3 and A1.



Step 08

After adjusting the variable points the patch looks like this:



Step 09

Now also condition 3 is satisfied, because couples of tile 1 and 2 can be translated horizontally and can form a stripe of infinite length between parallel straight lines. These stripes admit a tiling of the plane in infinitely many different ways.



Step 10

This is the most important step. Choose a point G1 on the line segment H1Z and cut off the green part of the nose of tile 1 (the angle 2ε at G1 is suggested, but variations are possible).
Next paste the green triangle at tile 2 without changing the position of the triangle. That means the area of the patch is not changed.
Tile has now a hook at its tail (take in account that tile 2 is oriented upside down).




Step 11

Of course we have to cut of the same tringle from tile 2 and 3. The triangle cutted from tile 2 is pasted to tile 1 und thus no hole occurs in the patch.



Step 12

The triangle cutted from the nose of tile 3 has to be moved and pasted correctly to the tail of the same tile. Afterwards all 3 tiles are congruent again.
Now also condition 2 is satisfied and we are done.



 Walter Trump, Nürnberg, Germany, ms(at)trump.de, © 2025-02-06 (last modified: 2025-03-10)