Portal:Mathematics/Featured picture/2009 01

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Picture of the month

Credit: Peo

In his historic work Elements, Euclid assumed the existence of parallel lines with his fifth postulate. His parallel postulate is equivalent to:

Given a line and a point not on that line, exactly one line can be drawn through that point which does not intersect the original line. (case 1)

In the 19th century mathematicians began to seriously question the parallel postulate and found that other forms of geometry are possible. For example in elliptical geometry:

Given a line and a point not on that line, all lines drawn through that point will intersect the original line. (case 2)

And in hyperbolic geometry:

Given a line and a point not on that line, an infinite number of lines can be drawn through the point that do not intersect the original line. (case 3)

These other forms of geometry, where the parallel postulate does not hold are called non-Euclidean geometries.

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