# Parallelogram Iterations

The applet below illustrates an astounding fact discovered by Fabian Rothelius.

Let there be a parallelogram ABCD. Starting with ABCD, Fabian's algorithm generates another parallelogram A'B'C'D'. The algorithm then iterates on the latest parallelogram. (The applet allows for 200 iterations.)

Here is how it works:

A' = (A + B)/2,

D' = (A + D)/2,

B' = A' + C - A,

C' = D' + C - A.

You can see how this works by increasing the number of iterations in the applet.

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A' = (A + B)/2,

D' = (A + D)/2,

B' = A' + C - A,

C' = D' + C - A.

Fabian gives the following explanation:

Consider a parallelogram where AD = √½AB, and where angle

However, every other parallelogram can be thought of as an affine transformation of this parallelogram. But the construction of A'B'C'D' is preserved by this transformation, so those vertices (and those of every subsequent parallelogram) lie on the images of the three concentric circles, which are ellipses with common axes.

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Copyright © 1996-2018 Alexander Bogomolny