Perspectives on the teaching of geometry for the 21st century. Ayuda Privacidad Condiciones. Geometry in nature VL Hansen. Some theorems on direct limits of expanding sequences of manifolds VL Hansen Mathematica Scandinavica 29, , Math , , Polynomial covering spaces and homomorphisms into the braid groups VL Hansen Pacific Journal of Mathematics 81 2 , , On the space of maps of a closed surface into the 2-sphere VL Hansen Mathematica Scandinavica 35 2 , , At the same time, the discovery of the method of single-focused perspective transformed first architecture and then the practice of painting, where it produced a dramatically heightened realism.

The technique proved eminently teachable, although few painters apart from Piero della Francesca c. Figure 2. Part of a surface of constant negative curvature. Girard Desargues — brought together projective ideas from both architecture and painting to create the first fully unified theory of conic sections all nondegenerate conic sections are projections of a circle. This theory naturally highlights those aspects that are projective such as tangency questions and it led directly or indirectly to a number of novel discoveries over the next century before it petered out.

In the form of simple horizontal and vertical projections it became the core technique of descriptive geometry or engineering drawing, a mainstay of French mathematical education throughout the nineteenth century, and, of course, it is still in use in the early twenty-first century.

## Projective Geometry

Poncelet's breakthrough at the start of the nineteenth century was to see that, for many geometric properties a curve is equivalent to any of its "shadows" its images under central projection. Although the details remained obscure for some time, the key point was that projective geometry discussed geometric properties of figures that do not involve the concept of distance.

Any theorem in projective geometry is true in Euclidean geometry, but not vice versa, and so projective geometry is more basic than Euclidean geometry. Figure 3.

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Part of a surface of constant positive curvature. Descartes took contemporary algebra, rewrote it in simpler notation, and proceeded to solve geometric problems by recasting them in algebraic terms and solving them by algebraic means, then reinterpreting the solution in geometric terms. Typically, the algebraic solution will be a single equation between two unknowns.

It is 1,3,0 or any multiple of 1,3,0. Since the third coordinate is zero, however, this is a point at infinity. In the Euclidean plane, the lines are parallel and do not. In the projective plane, they intersect at infinity. One can use this equation to find where a curve crosses the line at infinity.

### Projective geometry

Conic sections can be thought of as central projections of a circle. One can ask where, if at all, the projection of a circle crosses the line at infinity. In homogeneous coordinates it is. The parabola intersects the line at infinity at the single point 0,1,0. In other words it is tangent to the line at infinity.

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Projections do not keep distances constant, nor do they enlarge or shrink them in an obvious way. It is known as the cross ratio.

## Math Geometry Notes on Line

Projective geometry is the study of geometric properties which are not changed by a projective transformation. For example, the Sun shining behind a person projects his or her shadow onto the ground. Since the Sun's rays are for all practical purposes parallel, it is a parallel projection.

Since the rays of light pass through the slide, through the lens , and onto the screen, and since the lens acts like a point through which all the rays pass, it is a central projection. Some of the things that are or can be changed by a projection are size and angles. One's shadow is short in the middle of the day but very long toward sunset.

A pair of sticks which are crossed at right angles can cast shadows which are not at right angles. A painting is a central projection of the points in the scene onto a canvas or wall, with the artist's eye as the center of the projection the fact that the rays are converging on the artist's eye instead of emanating from it does not change the principles involved , but the scenes, usually Biblical, existed only in the artists' imagination. Among those who sought such principles was Gerard Desargues One of the many things he discovered was the remarkable theorem which now bears his name:.

If the two triangles were in separate planes, however, in which case the theorem is not only true, it is easier to prove one of the triangles could be a triangle on the ground and the other its projection on the artist's canvas. If one imagines a "point at infinity," however, they would intersect and the theorem would hold true.

Kepler is credited with introducing such an idea, but Desargues is credited with being the first to use it systematically. It is this principle that connects Desargues' theorem with its converse, although the connection is not obvious.

It is more apparent in the three postulates which Eves gives for projective geometry:. Even without drawings, one can note that writing "line" in place of "point" and vice versa results in a postulate that says just what it said before. One can also note that postulate I guarantees that every two lines will intersect, even lines which in Euclidean geometry would be parallel.

If one starts with an ordinary Euclidean plane in which points are addressed with Cartesian coordinates , x,y , this plane can be converted to a projective plane by adding a "line at infinity. One creates a point at infinity by making the third coordinate zero , for instance 4,1,0. Nevertheless it is a perfectly good projective point. It just happens to be "at infinity.

In homogeneous coordinates they do.