Collinearity

Last updated October 22, 2024

In geometry, collinearity of a set of points is the property of their lying on a single line.^[1] A set of points with this property is said to be collinear (sometimes spelled as colinear^[2]). In greater generality, the term has been used for aligned objects, that is, things being "in a line" or "in a row".

Points on a line

In any geometry, the set of points on a line are said to be collinear. In Euclidean geometry this relation is intuitively visualized by points lying in a row on a "straight line". However, in most geometries (including Euclidean) a line is typically a primitive (undefined) object type, so such visualizations will not necessarily be appropriate. A model for the geometry offers an interpretation of how the points, lines and other object types relate to one another and a notion such as collinearity must be interpreted within the context of that model. For instance, in spherical geometry, where lines are represented in the standard model by great circles of a sphere, sets of collinear points lie on the same great circle. Such points do not lie on a "straight line" in the Euclidean sense, and are not thought of as being in a row.

A mapping of a geometry to itself which sends lines to lines is called a collineation ; it preserves the collinearity property. The linear maps (or linear functions) of vector spaces, viewed as geometric maps, map lines to lines; that is, they map collinear point sets to collinear point sets and so, are collineations. In projective geometry these linear mappings are called homographies and are just one type of collineation.

Examples in Euclidean geometry

Triangles

In any triangle the following sets of points are collinear:

The orthocenter, the circumcenter, the centroid, the Exeter point, the de Longchamps point, and the center of the nine-point circle are collinear, all falling on a line called the Euler line.
The de Longchamps point also has other collinearities.
Any vertex, the tangency of the opposite side with an excircle, and the Nagel point are collinear in a line called a splitter of the triangle.
The midpoint of any side, the point that is equidistant from it along the triangle's boundary in either direction (so these two points bisect the perimeter), and the center of the Spieker circle are collinear in a line called a cleaver of the triangle. (The Spieker circle is the incircle of the medial triangle, and its center is the center of mass of the perimeter of the triangle.)
Any vertex, the tangency of the opposite side with the incircle, and the Gergonne point are collinear.
From any point on the circumcircle of a triangle, the nearest points on each of the three extended sides of the triangle are collinear in the Simson line of the point on the circumcircle.
The lines connecting the feet of the altitudes intersect the opposite sides at collinear points.^[3]^: p.199
A triangle's incenter, the midpoint of an altitude, and the point of contact of the corresponding side with the excircle relative to that side are collinear.^[4]^{: p.120, #78}
Menelaus' theorem states that three points $P_{1},P_{2},P_{3}$ on the sides (some extended) of a triangle opposite vertices $A_{1},A_{2},A_{3}$ respectively are collinear if and only if the following products of segment lengths are equal:^[3]^{: p. 147}

P_{1}A_{2}\cdot P_{2}A_{3}\cdot P_{3}A_{1}=P_{1}A_{3}\cdot P_{2}A_{1}\cdot P_{3}A_{2}.

The incenter, the centroid, and the Spieker circle's center are collinear.
The circumcenter, the Brocard midpoint, and the Lemoine point of a triangle are collinear.^[5]
Two perpendicular lines intersecting at the orthocenter of a triangle each intersect each of the triangle's extended sides. The midpoints on the three sides of these points of intersection are collinear in the Droz–Farny line.

Quadrilaterals

In a convex quadrilateral $ABCD$ whose opposite sides intersect at $E$ and $F$ , the midpoints of $AC, BD, EF$ are collinear and the line through them is called the Newton line. If the quadrilateral is a tangential quadrilateral, then its incenter also lies on this line.^[6]
In a convex quadrilateral, the quasiorthocenter $H$ , the "area centroid" $G$ , and the quasicircumcenter $O$ are collinear in this order, and $HG = 2 GO$ .^[7] (See Quadrilateral#Remarkable points and lines in a convex quadrilateral.)

Other collinearities of a tangential quadrilateral are given in Tangential quadrilateral#Collinear points.
In a cyclic quadrilateral, the circumcenter, the vertex centroid (the intersection of the two bimedians), and the anticenter are collinear.^[8]
In a cyclic quadrilateral, the area centroid, the vertex centroid, and the intersection of the diagonals are collinear.^[9]
In a tangential trapezoid, the tangencies of the incircle with the two bases are collinear with the incenter.
In a tangential trapezoid, the midpoints of the legs are collinear with the incenter.

Hexagons

Pascal's theorem (also known as the Hexagrammum Mysticum Theorem) states that if an arbitrary six points are chosen on a conic section (i.e., ellipse, parabola or hyperbola) and joined by line segments in any order to form a hexagon, then the three pairs of opposite sides of the hexagon (extended if necessary) meet in three points which lie on a straight line, called the Pascal line of the hexagon. The converse is also true: the Braikenridge–Maclaurin theorem states that if the three intersection points of the three pairs of lines through opposite sides of a hexagon lie on a line, then the six vertices of the hexagon lie on a conic, which may be degenerate as in Pappus's hexagon theorem.

Conic sections

By Monge's theorem, for any three circles in a plane, none of which is completely inside one of the others, the three intersection points of the three pairs of lines, each externally tangent to two of the circles, are collinear.
In an ellipse, the center, the two foci, and the two vertices with the smallest radius of curvature are collinear, and the center and the two vertices with the greatest radius of curvature are collinear.
In a hyperbola, the center, the two foci, and the two vertices are collinear.

Cones

The center of mass of a conic solid of uniform density lies one-quarter of the way from the center of the base to the vertex, on the straight line joining the two.

Tetrahedrons

The centroid of a tetrahedron is the midpoint between its Monge point and circumcenter. These points define the Euler line of the tetrahedron that is analogous to the Euler line of a triangle. The center of the tetrahedron's twelve-point sphere also lies on the Euler line.

Algebra

Collinearity of points whose coordinates are given

In coordinate geometry, in $n$ -dimensional space, a set of three or more distinct points are collinear if and only if, the matrix of the coordinates of these vectors is of rank 1 or less. For example, given three points

{\begin{aligned}X&=(x_{1},\ x_{2},\ \dots ,\ x_{n}),\\Y&=(y_{1},\ y_{2},\ \dots ,\ y_{n}),\\Z&=(z_{1},\ z_{2},\ \dots ,\ z_{n}),\end{aligned}}

if the matrix

{\begin{bmatrix}x_{1}&x_{2}&\dots &x_{n}\\y_{1}&y_{2}&\dots &y_{n}\\z_{1}&z_{2}&\dots &z_{n}\end{bmatrix}}

is of rank 1 or less, the points are collinear.

Equivalently, for every subset of $X, Y, Z$ , if the matrix

{\begin{bmatrix}1&x_{1}&x_{2}&\dots &x_{n}\\1&y_{1}&y_{2}&\dots &y_{n}\\1&z_{1}&z_{2}&\dots &z_{n}\end{bmatrix}}

is of rank 2 or less, the points are collinear. In particular, for three points in the plane ( $n = 2$ ), the above matrix is square and the points are collinear if and only if its determinant is zero; since that 3 × 3 determinant is plus or minus twice the area of a triangle with those three points as vertices, this is equivalent to the statement that the three points are collinear if and only if the triangle with those points as vertices has zero area.

Collinearity of points whose pairwise distances are given

A set of at least three distinct points is called straight, meaning all the points are collinear, if and only if, for every three of those points $A, B, C$ , the following determinant of a Cayley–Menger determinant is zero (with $d (AB)$ meaning the distance between $A$ and $B$ , etc.):

\det {\begin{bmatrix}0&d(AB)^{2}&d(AC)^{2}&1\\d(AB)^{2}&0&d(BC)^{2}&1\\d(AC)^{2}&d(BC)^{2}&0&1\\1&1&1&0\end{bmatrix}}=0.

This determinant is, by Heron's formula, equal to −16 times the square of the area of a triangle with side lengths $d (AB), d (BC), d (AC)$ ; so checking if this determinant equals zero is equivalent to checking whether the triangle with vertices $A, B, C$ has zero area (so the vertices are collinear).

Equivalently, a set of at least three distinct points are collinear if and only if, for every three of those points $A, B, C$ with $d (AC)$ greater than or equal to each of $d (AB)$ and $d (BC)$ , the triangle inequality $d (AC) \leq d (AB) + d (BC)$ holds with equality.

Number theory

Two numbers $m$ and $n$ are not coprime —that is, they share a common factor other than 1—if and only if for a rectangle plotted on a square lattice with vertices at $(0, 0), (m, 0), (m, n), (0, n)$ , at least one interior point is collinear with $(0, 0)$ and $(m, n)$ .

Concurrency (plane dual)

In various plane geometries the notion of interchanging the roles of "points" and "lines" while preserving the relationship between them is called plane duality. Given a set of collinear points, by plane duality we obtain a set of lines all of which meet at a common point. The property that this set of lines has (meeting at a common point) is called concurrency, and the lines are said to be concurrent lines. Thus, concurrency is the plane dual notion to collinearity.

Collinearity graph

Given a partial geometry $P$ , where two points determine at most one line, a collinearity graph of $P$ is a graph whose vertices are the points of $P$ , where two vertices are adjacent if and only if they determine a line in $P$ .

Usage in statistics and econometrics

In statistics, collinearity refers to a linear relationship between two explanatory variables. Two variables are perfectly collinear if there is an exact linear relationship between the two, so the correlation between them is equal to 1 or −1. That is, $X 1$ and $X 2$ are perfectly collinear if there exist parameters $\lambda _{0}$ and $\lambda _{1}$ such that, for all observations $i$ , we have

X_{2i}=\lambda _{0}+\lambda _{1}X_{1i}.

This means that if the various observations $(X 1 i, X 2 i)$ are plotted in the $(X 1, X 2)$ plane, these points are collinear in the sense defined earlier in this article.

Perfect multicollinearity refers to a situation in which $k (k \geq 2)$ explanatory variables in a multiple regression model are perfectly linearly related, according to

X_{ki}=\lambda _{0}+\lambda _{1}X_{1i}+\lambda _{2}X_{2i}+\dots +\lambda _{k-1}X_{(k-1),i}

for all observations $i$ . In practice, we rarely face perfect multicollinearity in a data set. More commonly, the issue of multicollinearity arises when there is a "strong linear relationship" among two or more independent variables, meaning that

X_{ki}=\lambda _{0}+\lambda _{1}X_{1i}+\lambda _{2}X_{2i}+\dots +\lambda _{k-1}X_{(k-1),i}+\varepsilon _{i}

where the variance of $\varepsilon _{i}$ is relatively small.

The concept of lateral collinearity expands on this traditional view, and refers to collinearity between explanatory and criteria (i.e., explained) variables.^[10]

Usage in other areas

Antenna arrays

An antenna mast with four collinear directional arrays. Antenna-tower-collinear-et-al.jpg — An antenna mast with four collinear directional arrays.

In telecommunications, a collinear (or co-linear) antenna array is an array of dipole antennas mounted in such a manner that the corresponding elements of each antenna are parallel and aligned, that is they are located along a common line or axis.

Photography

The collinearity equations are a set of two equations, used in photogrammetry and computer stereo vision, to relate coordinates in an image (sensor) plane (in two dimensions) to object coordinates (in three dimensions). In the photography setting, the equations are derived by considering the central projection of a point of the object through the optical centre of the camera to the image in the image (sensor) plane. The three points, object point, image point and optical centre, are always collinear. Another way to say this is that the line segments joining the object points with their image points are all concurrent at the optical centre.^[11]

Notes

↑ The concept applies in any geometry Dembowski (1968 , pg. 26), but is often only defined within the discussion of a specific geometry Coxeter (1969 , pg. 178), Brannan, Esplen & Gray (1998 , pg.106)
↑ Colinear (Merriam-Webster dictionary)
1 2 Johnson, Roger A., Advanced Euclidean Geometry, Dover Publ., 2007 (orig. 1929).
↑ Altshiller Court, Nathan. College Geometry, 2nd ed. Barnes & Noble, 1952 [1st ed. 1925].
↑ Scott, J. A. "Some examples of the use of areal coordinates in triangle geometry", Mathematical Gazette 83, November 1999, 472–477.
↑ Dušan Djukić, Vladimir Janković, Ivan Matić, Nikola Petrović, The IMO Compendium, Springer, 2006, p. 15.
↑ Myakishev, Alexei (2006), "On Two Remarkable Lines Related to a Quadrilateral" (PDF), Forum Geometricorum, 6: 289–295.
↑ Honsberger, Ross (1995), "4.2 Cyclic quadrilaterals", Episodes in Nineteenth and Twentieth Century Euclidean Geometry, New Mathematical Library, vol. 37, Cambridge University Press, pp. 35–39, ISBN 978-0-88385-639-0
↑ Bradley, Christopher (2011), Three Centroids created by a Cyclic Quadrilateral (PDF)
↑ Kock, N.; Lynn, G. S. (2012). "Lateral collinearity and misleading results in variance-based SEM: An illustration and recommendations" (PDF). Journal of the Association for Information Systems. 13 (7): 546–580. doi:10.17705/1jais.00302. S2CID 3677154.
↑ It's more mathematically natural to refer to these equations as concurrency equations, but photogrammetry literature does not use that terminology.

Related Research Articles

In geometry a quadrilateral is a four-sided polygon, having four edges (sides) and four corners (vertices). The word is derived from the Latin words quadri, a variant of four, and latus, meaning "side". It is also called a tetragon, derived from Greek "tetra" meaning "four" and "gon" meaning "corner" or "angle", in analogy to other polygons. Since "gon" means "angle", it is analogously called a quadrangle, or 4-angle. A quadrilateral with vertices $,, and is sometimes denoted as .$

A triangle is a polygon with three corners and three sides, one of the basic shapes in geometry. The corners, also called vertices, are zero-dimensional points while the sides connecting them, also called edges, are one-dimensional line segments. A triangle has three internal angles, each one bounded by a pair of adjacent edges; the sum of angles of a triangle always equals a straight angle. The triangle is a plane figure and its interior is a planar region. Sometimes an arbitrary edge is chosen to be the base, in which case the opposite vertex is called the apex; the shortest segment between base and apex is the height. The area of a triangle equals one-half the product of height and base length.

In Euclidean geometry, Ceva's theorem is a theorem about triangles. Given a triangle $△ ABC$ , let the lines $AO, BO, CO$ be drawn from the vertices to a common point $O$ , to meet opposite sides at $D, E, F$ respectively. Then, using signed lengths of segments,

<span class="mw-page-title-main">Bisection</span> Division of something into two equal or congruent parts

In geometry, bisection is the division of something into two equal or congruent parts. Usually it involves a bisecting line, also called a bisector. The most often considered types of bisectors are the segment bisector, a line that passes through the midpoint of a given segment, and the angle bisector, a line that passes through the apex of an angle . In three-dimensional space, bisection is usually done by a bisecting plane, also called the bisector.

<span class="mw-page-title-main">Centroid</span> Mean position of all the points in a shape

In mathematics and physics, the centroid, also known as geometric center or center of figure, of a plane figure or solid figure is the arithmetic mean position of all the points in the surface of the figure. The same definition extends to any object in $-dimensional Euclidean space.$

<span class="mw-page-title-main">Euler line</span> Line constructed from a triangle

In geometry, the Euler line, named after Leonhard Euler, is a line determined from any triangle that is not equilateral. It is a central line of the triangle, and it passes through several important points determined from the triangle, including the orthocenter, the circumcenter, the centroid, the Exeter point and the center of the nine-point circle of the triangle.

In mathematics, an affine space is a geometric structure that generalizes some of the properties of Euclidean spaces in such a way that these are independent of the concepts of distance and measure of angles, keeping only the properties related to parallelism and ratio of lengths for parallel line segments. Affine space is the setting for affine geometry.

<span class="mw-page-title-main">Incenter</span> Center of the inscribed circle of a triangle

In geometry, the incenter of a triangle is a triangle center, a point defined for any triangle in a way that is independent of the triangle's placement or scale. The incenter may be equivalently defined as the point where the internal angle bisectors of the triangle cross, as the point equidistant from the triangle's sides, as the junction point of the medial axis and innermost point of the grassfire transform of the triangle, and as the center point of the inscribed circle of the triangle.

In geometry, the cross-ratio, also called the double ratio and anharmonic ratio, is a number associated with a list of four collinear points, particularly points on a projective line. Given four points $A$ , $B$ , $C$ , $D$ on a line, their cross ratio is defined as

<span class="mw-page-title-main">Midpoint</span> Point on a line segment which is equidistant from both endpoints

In geometry, the midpoint is the middle point of a line segment. It is equidistant from both endpoints, and it is the centroid both of the segment and of the endpoints. It bisects the segment.

<span class="mw-page-title-main">Cubic plane curve</span> Type of a mathematical curve

In mathematics, a cubic plane curve is a plane algebraic curve $C$ defined by a cubic equation

<span class="mw-page-title-main">Barycentric coordinate system</span> Coordinate system that is defined by points instead of vectors

In geometry, a barycentric coordinate system is a coordinate system in which the location of a point is specified by reference to a simplex. The barycentric coordinates of a point can be interpreted as masses placed at the vertices of the simplex, such that the point is the center of mass of these masses. These masses can be zero or negative; they are all positive if and only if the point is inside the simplex.

<span class="mw-page-title-main">Line (geometry)</span> Straight figure with zero width and depth

In geometry, a straight line, usually abbreviated line, is an infinitely long object with no width, depth, or curvature, an idealization of such physical objects as a straightedge, a taut string, or a ray of light. Lines are spaces of dimension one, which may be embedded in spaces of dimension two, three, or higher. The word line may also refer, in everyday life, to a line segment, which is a part of a line delimited by two points.

In geometry, the circumscribed circle or circumcircle of a triangle is a circle that passes through all three vertices. The center of this circle is called the circumcenter of the triangle, and its radius is called the circumradius. The circumcenter is the point of intersection between the three perpendicular bisectors of the triangle's sides, and is a triangle center.

In Euclidean geometry, the medial triangle or midpoint triangle of a triangle $△ ABC$ is the triangle with vertices at the midpoints of the triangle's sides $AB, AC, BC$ . It is the $n = 3$ case of the midpoint polygon of a polygon with $n$ sides. The medial triangle is not the same thing as the median triangle, which is the triangle whose sides have the same lengths as the medians of $△ ABC$ .

In geometry, the trilinear coordinates $x : y : z$ of a point relative to a given triangle describe the relative directed distances from the three sidelines of the triangle. Trilinear coordinates are an example of homogeneous coordinates. The ratio $x : y$ is the ratio of the perpendicular distances from the point to the sides opposite vertices $A$ and $B$ respectively; the ratio $y : z$ is the ratio of the perpendicular distances from the point to the sidelines opposite vertices $B$ and $C$ respectively; and likewise for $z : x$ and vertices $C$ and $A$ .

In Euclidean geometry, a tangential quadrilateral or circumscribed quadrilateral is a convex quadrilateral whose sides all can be tangent to a single circle within the quadrilateral. This circle is called the incircle of the quadrilateral or its inscribed circle, its center is the incenter and its radius is called the inradius. Since these quadrilaterals can be drawn surrounding or circumscribing their incircles, they have also been called circumscribable quadrilaterals, circumscribing quadrilaterals, and circumscriptible quadrilaterals. Tangential quadrilaterals are a special case of tangential polygons.

In Euclidean geometry, a circumconic is a conic section that passes through the three vertices of a triangle, and an inconic is a conic section inscribed in the sides, possibly extended, of a triangle.

<span class="mw-page-title-main">Projective harmonic conjugate</span> Point found separated from another, given a point pair

In projective geometry, the harmonic conjugate point of a point on the real projective line with respect to two other points is defined by the following construction:

In Euclidean geometry, a triangle conic is a conic in the plane of the reference triangle and associated with it in some way. For example, the circumcircle and the incircle of the reference triangle are triangle conics. Other examples are the Steiner ellipse, which is an ellipse passing through the vertices and having its centre at the centroid of the reference triangle; the Kiepert hyperbola which is a conic passing through the vertices, the centroid and the orthocentre of the reference triangle; and the Artzt parabolas, which are parabolas touching two sidelines of the reference triangle at vertices of the triangle.

References

Brannan, David A.; Esplen, Matthew F.; Gray, Jeremy J. (1998), Geometry, Cambridge University Press, ISBN 0-521-59787-0
Coxeter, H. S. M. (1969), Introduction to Geometry, New York: John Wiley & Sons, ISBN 0-471-50458-0
Dembowski, Peter (1968), Finite geometries , Ergebnisse der Mathematik und ihrer Grenzgebiete, vol. 44, Berlin: Springer, ISBN 3-540-61786-8, MR 0233275

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[1] The concept applies in any geometry Dembowski (1968 , pg. 26), but is often only defined within the discussion of a specific geometry Coxeter (1969 , pg. 178), Brannan, Esplen & Gray (1998 , pg.106)

[2] Colinear (Merriam-Webster dictionary)

[Johnson-3] 1 2 Johnson, Roger A., Advanced Euclidean Geometry, Dover Publ., 2007 (orig. 1929).

[AC-4] Altshiller Court, Nathan. College Geometry, 2nd ed. Barnes & Noble, 1952 [1st ed. 1925].

[5] Scott, J. A. "Some examples of the use of areal coordinates in triangle geometry", Mathematical Gazette 83, November 1999, 472–477.

[6] Dušan Djukić, Vladimir Janković, Ivan Matić, Nikola Petrović, The IMO Compendium, Springer, 2006, p. 15.

[Myakishev-7] Myakishev, Alexei (2006), "On Two Remarkable Lines Related to a Quadrilateral" (PDF), Forum Geometricorum, 6: 289–295.

[Honsberger-8] Honsberger, Ross (1995), "4.2 Cyclic quadrilaterals", Episodes in Nineteenth and Twentieth Century Euclidean Geometry, New Mathematical Library, vol. 37, Cambridge University Press, pp. 35–39, ISBN 978-0-88385-639-0

[9] Bradley, Christopher (2011), Three Centroids created by a Cyclic Quadrilateral (PDF)

[10] Kock, N.; Lynn, G. S. (2012). "Lateral collinearity and misleading results in variance-based SEM: An illustration and recommendations" (PDF). Journal of the Association for Information Systems. 13 (7): 546–580. doi:10.17705/1jais.00302. S2CID 3677154.

[11] It's more mathematically natural to refer to these equations as concurrency equations, but photogrammetry literature does not use that terminology.

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