Similarity (geometry)

Other similar polygons

The concept of similarity extends to polygons with more than three sides. Given any two similar polygons, corresponding sides taken in the same sequence (even if clockwise for one polygon and counterclockwise for the other) are proportional and corresponding angles taken in the same sequence are equal in measure. However, proportionality of corresponding sides is not by itself sufficient to prove similarity for polygons beyond triangles (otherwise, for example, all rhombi would be similar). Likewise, equality of all angles in sequence is not sufficient to guarantee similarity (otherwise all rectangles would be similar). A sufficient condition for similarity of polygons is that corresponding sides and diagonals are proportional.

For given n, all regular n-gons are similar.

Similar curves

Several types of curves have the property that all examples of that type are similar to each other. These include:

• Lines (any two lines are even congruent)
• Line segments
• Circles
• Parabolas
• Hyperbolas of a specific eccentricity
• Ellipses of a specific eccentricity
• Catenaries
• Graphs of the logarithm function for different bases
• Graphs of the exponential function for different bases
• Logarithmic spirals are self-similar

In Euclidean space

A similarity (also called a similarity transformation or similitude) of a Euclidean space is a bijection f from the space onto itself that multiplies all distances by the same positive real number r, so that for any two points x and y we have $${\displaystyle d(f(x),f(y))=rd(x,y),}$$ where d(x,y) is the Euclidean distance from x to y. The scalar r has many names in the literature, including the ratio of similarity, the stretching factor and the similarity coefficient. When r = 1 a similarity is called an isometry (rigid transformation). Two sets are called similar if one is the image of the other under a similarity.

As a map ⁠${\displaystyle f:{\mathbb{R}}^n\to {\mathbb{R}}^n,}$⁠ a similarity of ratio r takes the form $${\displaystyle f(x)=rAx+t,}$$ where ⁠${\displaystyle A\in O^n(\mathbb{R})}$⁠ is an n × n orthogonal matrix and ⁠${\displaystyle t\in {\mathbb{R}}^n}$⁠ is a translation vector.

Similarities preserve planes, lines, perpendicularity, parallelism, midpoints, inequalities between distances and line segments. Similarities preserve angles but do not necessarily preserve orientation, direct similitudes preserve orientation and opposite similitudes change it.

The similarities of Euclidean space form a group under the operation of composition called the similarities group S. The direct similitudes form a normal subgroup of S and the Euclidean group E(n) of isometries also forms a normal subgroup. The similarities group S is itself a subgroup of the affine group, so every similarity is an affine transformation.

One can view the Euclidean plane as the complex plane, that is, as a 2-dimensional space over the reals. The 2D similarity transformations can then be expressed in terms of complex arithmetic and are given by

• ${\displaystyle f(z)=az+b}$ (direct similitudes), and
• ${\displaystyle f(z)=a\overline{z}+b}$ (opposite similitudes),

where a and b are complex numbers, a ≠ 0. When |a|= 1, these similarities are isometries.

Area ratio and volume ratio

The ratio between the areas of similar figures is equal to the square of the ratio of corresponding lengths of those figures (for example, when the side of a square or the radius of a circle is multiplied by three, its area is multiplied by nine — i.e. by three squared). The altitudes of similar triangles are in the same ratio as corresponding sides. If a triangle has a side of length b and an altitude drawn to that side of length h then a similar triangle with corresponding side of length kb will have an altitude drawn to that side of length kh. The area of the first triangle is ${\displaystyle A=\frac{1}{2}bh,}$ while the area of the similar triangle will be $${\displaystyle A^{\prime }=\frac{1}{2}\cdot kb\cdot kh=k^{2}A.}$$ Similar figures which can be decomposed into similar triangles will have areas related in the same way. The relationship holds for figures that are not rectifiable as well.

The ratio between the volumes of similar figures is equal to the cube of the ratio of corresponding lengths of those figures (for example, when the edge of a cube or the radius of a sphere is multiplied by three, its volume is multiplied by 27 — i.e. by three cubed).

Galileo's square–cube law concerns similar solids. If the ratio of similitude (ratio of corresponding sides) between the solids is k, then the ratio of surface areas of the solids will be k², while the ratio of volumes will be k³.

Similarity with a center

If a similarity has exactly one invariant point: a point that the similarity keeps unchanged, then this only point is called "center" of the similarity.

On the first image below the title, on the left, one or another similarity shrinks a regular polygon into a concentric one, the vertices of which are each on a side of the previous polygon. This rotational reduction is repeated, so the initial polygon is extended into an abyss of regular polygons. The center of the similarity is the common center of the successive polygons. A red segment joins a vertex of the initial polygon to its image under the similarity, followed by a red segment going to the following image of vertex, and so on to form a spiral. Actually we can see more than three direct similarities on this first image, because every regular polygon is invariant under certain direct similarities, more precisely certain rotations the center of which is the center of the polygon, and a composition of direct similarities is also a direct similarity. For example we see the image of the initial regular pentagon under a homothety of negative ratio −k, which is a similarity of ±180° angle and a positive ratio equal to k.

Below the title on the right, the second image shows a similarity decomposed into a rotation and a homothety. Similarity and rotation have the same angle of +135 degrees modulo 360 degrees. Similarity and homothety have the same ratio of ⁠${\displaystyle \frac{\sqrt{2}}{2},}$⁠ multiplicative inverse of the ratio ⁠${\displaystyle \sqrt{2}}$⁠ (square root of 2) of the inverse similarity. Point S is the common center of the three transformations: rotation, homothety and similarity. For example point W is the image of F under the rotation, and point T is the image of W under the homothety, more briefly $${\displaystyle T=H(W)=(R(F))=(H\circ R)(F)=D(F),}$$ by naming R, H and D the previous rotation, homothety and similarity, with “D" like "Direct".

This direct similarity that transforms triangle △EFA into triangle △ATB can be decomposed into a rotation and a homothety of same center S in several manners. For example, D = R ○ H = H ○ R, the last decomposition being only represented on the image. To get D we can also compose in any order a rotation of −45° angle and a homothety of ratio ⁠${\displaystyle \frac{-\sqrt{2}}{2}.}$⁠

With "M" like "Mirror" and "I" like "Indirect", if M is the reflection with respect to line CW, then M ○ D = I is the indirect similarity that transforms segment BF like D into segment CT, but transforms point E into B and point A into A itself. Square ACBT is the image of ABEF under similarity I of ratio ⁠${\displaystyle \frac{1}{\sqrt{2}}.}$⁠ Point A is the center of this similarity because any point K being invariant under it fulfills ${\displaystyle AK=\frac{AK}{\sqrt{2}},}$ only possible if AK = 0, otherwise written A = K.

How to construct the center S of direct similarity D from square ABEF, how to find point S center of a rotation of +135° angle that transforms ray ⁠${\displaystyle \stackrel{}{\overrightarrow{SE}}}$⁠ into ray ⁠${\displaystyle \stackrel{}{\overrightarrow{SA}}}$⁠? This is an inscribed angle problem plus a question of orientation. The set of points P such that ${\displaystyle \stackrel{}{\overrightarrow{PE},\overrightarrow{PA}=+{135}^{\circ }}}$ is an arc of circle EA that joins E and A, of which the two radius leading to E and A form a central angle of 2(180° − 135°) = 2 × 45° = 90°. This set of points is the blue quarter of circle of center F inside square ABEF. In the same manner, point S is a member of the blue quarter of circle of center T inside square BCAT. So point S is the intersection point of these two quarters of circles.

In general metric spaces

A bijective map ${\displaystyle f:M\to N}$ between metric spaces is called a similarity if ${\displaystyle d(f(x),f(y))=c\cdot d(x,y)}$ for all ${\displaystyle x,y\in M}$ where ${\displaystyle c>0}$.

Weaker versions of similarity would for instance have f be a bi-Lipschitz function and the scalar r a limit

$${\displaystyle lim\frac{d(f(x),f(y))}{d(x,y)}=r.}$$

This weaker version applies when the metric is an effective resistance on a topologically self-similar set.

A self-similar subset of a metric space (X, d) is a set K for which there exists a finite set of similitudes { f_s}_s∈S with contraction factors 0 ≤ r_s < 1 such that K is the unique compact subset of X for which

$${\displaystyle \bigcup _{s\in S}{f}_s(K)=K.}$$

These self-similar sets have a self-similar measure μ^D with dimension D given by the formula

$${\displaystyle \sum _{s\in S}(r_s{)}^D=1}$$

which is often (but not always) equal to the set's Hausdorff dimension and packing dimension. If the overlaps between the f_s(K) are "small", we have the following simple formula for the measure:

$${\displaystyle {\mu }^D(f_{s_1}\circ f_{s_2}\circ \cdots \circ f_{s_n}(K))=(r_{s_1}\cdot r_{s_2}\cdots r_{s_n}{)}^D.}$$

Topology

In topology, a metric space can be constructed by defining a similarity instead of a distance. The similarity is a function such that its value is greater when two points are closer (contrary to the distance, which is a measure of dissimilarity: the closer the points, the lesser the distance).

The definition of the similarity can vary among authors, depending on which properties are desired. The basic common properties are

1. Positive defined:

$${\displaystyle \mathrm{\forall }(a,b),S(a,b)\ge 0}$$

1. Majored by the similarity of one element on itself (auto-similarity):

$${\displaystyle S(a,b)\le S(a,a)\text{and}\mathrm{\forall }(a,b),S(a,b)=S(a,a)\iff a=b}$$

More properties can be invoked, such as:

• Reflectivity: ${\displaystyle \mathrm{\forall }(a,b)S(a,b)=S(b,a),}$ or
• Finiteness: ${\displaystyle \mathrm{\forall }(a,b)S(a,b)<\mathrm{\infty }.}$

The upper value is often set at 1 (creating a possibility for a probabilistic interpretation of the similitude).

Note that, in the topological sense used here, a similarity is a kind of measure. This usage is not the same as the similarity transformation of the § In Euclidean space and § In general metric spaces sections of this article.

Psychology

The intuition for the notion of geometric similarity already appears in human children, as can be seen in their drawings. Certain perceptual categorization models in psychology are based on geometric similarity, assuming that learning involves the storage of specific instances (i.e., of general object specifications) in memory. The categorization of another object is subsequently based on the similarity of the object to the instances in memory.