# Line at infinity

:"Ideal line" redirects here. For the ideal line in racing, see

Racing line ."Ingeometry andtopology , the**line at infinity**is a line which is added to the real (affine) plane in order to give closure to, and remove the exceptional cases from, the incidence properties of the resultingprojective plane . The line at infinity is also called the**"ideal line**".**Geometric formulation**In projective geometry, any pair of lines always intersect at some point. But parallel lines do not intersect in the real plane. The line at

infinity is added to the real plane. This completes the plane, because now parallel lines intersect at a point which lies on the line at infinity. The point at which the parallel lines intersect depends only on theslope of the lines, not at all on theiry-intercept . Also, if any pair of lines intersect at a point on the line at infinity, then the pair of lines is parallel.Every line intersects the line at infinity at some point. The point at which a line intersects the line at infinity determines the slope of the line, but not at all its y-intercept.

In the affine plane, a line runs off in two opposite directions. In the projective plane, the two opposite directions of a line meet each other at a point on the line at infinity. Therefore lines in the projective plane are

closed curve s: they are cyclical rather than linear. This is true of the line at infinity itself: it meets itself at its two endpoints (which are therefore not endpoints at all) and so it is actually cyclical.**Topological perspective**The line at infinity can be visualized as a circle which surrounds the affine plane. However, this circle is actually like

cross-cap , which ishomeomorphic to aMöbius strip : diametrically opposite points of the circle are equivalent -- they are the same point. The combination of affine plane and line at infinity makes thereal projective plane , $mathbb\{R\}P^2$ .A

hyperbola can be seen as a closed curve which intersects the line at infinity in two different points. These two points are specified by the slopes of the twoasymptote s of the hyperbola. Likewise, aparabola can be seen as a closed curve which intersects the line at infinity in a single point. This point is specified by the slope of the axis of the parabola. If the parabola is cut by its vertex into a symmetrical pair of "horns", then these two horns become more parallel to each other further away from the vertex, and are actually parallel to the axis and to each other at infinity, so that they intersect at the line at infinity.The analogue for the complex projective plane is a 'line' at infinity that is (naturally) a complex

projective line . Topologically this is quite different, in that it is aRiemann sphere , which is therefore a 2-sphere , being added to a complex affine space of two dimensions over "C" (so four "real" dimensions), resulting in a four-dimensionalcompact manifold . The result isorientable , while the real projective plane is not.**History**The complex line at infinity was much used in

nineteenth century geometry. In fact one of the most applied tricks was to regard a circle as aconic constrained to pass through two points at infinity, the solutions of:"X"

^{2}+ "Y"^{2}= 0.This equation is the form taken by that of any circle when we drop terms of lower order in "X" and "Y". More formally, we should use

homogeneous coordinates : ["X:Y:Z"]

and note that the line at infinity is specified by setting

: "Z" = 0.

Making equations homogeneous by introducing powers of "Z", and then setting "Z" = 0, does precisely kill off terms of lower order.

Solving the equation, therefore, we find that all circles 'pass through' the "

circular points at infinity ":"I" = [1:"i":0] and "J" = [1:−"i":0] .

These of course are complex points, for any representing set of homogeneous coordinates. Since the projective plane has a large enough

symmetry group , they are in no way special, though. The conclusion is that the three-parameter family of circles can be treated as a special case of the linear system of conics passing through two given distinct points "P" and "Q". This idea was used so often that a schoolmasterly joke arose, naming the circular points at infinity "Isaac" and "Jacob", respectively.**ee also***

point at infinity

*plane at infinity

*hyperplane at infinity **References*** Casey, J., "A Sequel to the First Six Books of the Elements of Euclid, Containing an Easy Introduction to Modern Geometry with Numerous Examples", 5th ed., rev. enl. Dublin: Hodges, Figgis, & Co., 1888

* Kimberling, C., "Triangle Centers and Central Triangles." Congr. Numer. 129, 1-295, 1998

* Lachlan, R., "An Elementary Treatise on Modern Pure Geometry", sect. 10. London, Macmillian, p. 6, 1893

* Graustein, W. C., "Introduction to Higher Geometry". New York, Macmillan, p. 30, 1930

* Oldknow, A., "The Euler-Gergonne-Soddy Triangle of a Triangle." Amer. Math. Monthly 103, 319-329, 1996

* Wells, D., "The Penguin Dictionary of Curious and Interesting Geometry". London, Penguin, pp. 141-142, 1991

*Wikimedia Foundation.
2010.*

### Look at other dictionaries:

**Infinity**— In mathematics, infinity is often used in contexts where it is treated as if it were a number (i.e., it counts or measures things: an infinite number of terms ) but it is a different type of number from the real numbers. Infinity is related to… … Wikipedia**Line coordinates**— In geometry, line coordinates are used to specify the position of a line just as point coordinates (or simply coordinates) are used to specify the position of a point. Contents 1 Lines in the plane 2 Tangential equations 3 Tangential equation of… … Wikipedia**Infinity**— • The infinite, as the word indicates, is that which has no end, no limit, no boundary, and therefore cannot be measured by a finite standard, however often applied; it is that which cannot be attained by successive addition, not exhausted by… … Catholic encyclopedia**Infinity (audio)**— Infinity is a manufacturer of loudspeakers. Founded in 1968, Infinity has produced affordable home and mobile/car audio solutions by innovating materials used in loudspeaker production, such as neodymium magnets, and polypropylene cone and… … Wikipedia**Infinity**— In*fin i*ty, n.; pl. {Infinities}. [L. infinitas; pref. in not + finis boundary, limit, end: cf. F. infinit[ e]. See {Finite}.] [1913 Webster] 1. Unlimited extent of time, space, or quantity; eternity; boundlessness; immensity. Sir T. More. [1913 … The Collaborative International Dictionary of English**Infinity (wargame)**— Infinity is a science fiction skirmish level tabletop miniature wargame produced by Corvus Belli. Set about 175 years into the future, players of the game enact a skirmish between opposing forces using 28 mm scale miniatures and utilizing a… … Wikipedia**Infinity Fluids**— Infinity Fluids, founded in 1998, is a Massachusetts corporation which develops process and thermal systems for the fuel cell, pharmaceutical, industrial, and biotechnology industries.It was founded by Robert S. Evans and is a manufacturing and… … Wikipedia**Line Up (TV Show)**— Line Up is a Korean prime time reality television series produced and broadcasted by SBS in late 2007 and broadcasted every Saturday 6:30pm (KST). OverviewLine Up ( ko. 라인업) is a Korean television entertainment program, distributed by SBS. As of… … Wikipedia**infinity**— /in fin i tee/, n., pl. infinities. 1. the quality or state of being infinite. 2. something that is infinite. 3. infinite space, time, or quantity. 4. an infinite extent, amount, or number. 5. an indefinitely great amount or number. 6. Math. a.… … Universalium**Infinity on High**— Infinity on High … Wikipedia