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The existence of parallel lines leads to establishing a point at infinity which represents the intersection of these parallels. This axiomatic symmetry grew out of a study of graphical perspective where a parallel projection arises as a central projection where the center C is a point at infinity, or figurative point. [5]
In hyperbolic geometry, an ideal point, omega point [1] or point at infinity is a well-defined point outside the hyperbolic plane or space. Given a line l and a point P not on l, right- and left-limiting parallels to l through P converge to l at ideal points. Unlike the projective case, ideal points form a boundary, not a submanifold. So, these ...
More precisely, the point at infinity is the limit of every sequence of real numbers whose absolute values are increasing and unbounded. The projectively extended real line may be identified with a real projective line in which three points have been assigned the specific values 0 , 1 and ∞ .
The line at infinity is thus a line like any other in the theory: it is in no way special or distinguished. (In the later spirit of the Erlangen programme one could point to the way the group of transformations can move any line to the line at infinity). The parallel properties of elliptic, Euclidean and hyperbolic geometries contrast as follows:
The points at infinity are the "extra" points where parallel lines intersect in the construction of the extended real plane; the point (0, x 1, x 2) is where all lines of slope x 2 / x 1 intersect. Consider for example the two lines
In this usage, infinity is a mathematical concept, and infinite mathematical objects can be studied, manipulated, and used just like any other mathematical object. The mathematical concept of infinity refines and extends the old philosophical concept, in particular by introducing infinitely many different sizes of infinite sets.
In the limit, as approaches infinity, in other words, as the point moves away from the origin, approaches and the homogeneous coordinates of the point become (,,). Thus we define ( m , − n , 0 ) {\displaystyle (m,-n,0)} as the homogeneous coordinates of the point at infinity corresponding to the direction of the line n x + m y = 0 ...
The infinity symbol may also be used to represent a point at infinity, especially when there is only one such point under consideration. This usage includes, in particular, the infinite point of a projective line, [13] and the point added to a topological space to form its one-point compactification. [14]