# Homothety - an Affine Transform

*Homothety*, *dilation*, *central similarity* are all interchangeable terms used to describe a geometric transformation H_{O, k} defined by a point O called the *center of homothety* and a real number k, known as its *coefficient*. The case _{O, k}(P)

(1) | OP'/OP = k, |

where OP' and OP are considered as signed segments. Thus, for example, when k is negative P and P' are located on different sides of the center O.

(In the applet below, various Homotheties are controlled by a hollow blue point - the center of homothety, and a slider with three draggable points. The yellow and red points set 0 and 1 on the axis. The blue point defines the coefficient of the homothety relative to 0 and 1.)

What if applet does not run? |

The following observations are noteworthy:

Under a homothety, parallel lines map onto parallel lines.

Homothety preserves angles.

Homothety preserves orientation.

Every homothety has an inverse, viz., H

_{O, k}^{-1}= H_{O, 1/k}. In other wordsH _{O, k}(H_{O, 1/k}(P)) = P = H_{O, 1/k}(H_{O, k}(P)).

Every homothety with k different from 1 has one and only one fixed point - the center O. Every line through O is also fixed although not pointwise. For

k = 1, homothety is the identity transformation.Successive applications of homotheties with coefficients k

_{1}and k_{2}is either a homothety with coefficient k_{1}k_{2}, if the latter differs from 1, or a parallel translation, otherwise. In the former case, the centers of the three homotheties are collinear. In the latter case, the translation is parallel to the line joining the centers of the two homotheties.The product of homotheties is not in general commutative. Two homotheties with a common center commute as a matter of course.

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