Linear Algebra/Topic: Projective Geometry/Solutions

Solutions

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Problem 1

What is the equation of this point?

 
Answer

From the dot product

 

we get that the equation is  .

Problem 2
  1. Find the line incident on these points in the projective plane.
     
  2. Find the point incident on both of these projective lines.
     
Answer
  1. This determinant
     
    shows that the line is  .
  2.  
Problem 3

Find the formula for the line incident on two projective points. Find the formula for the point incident on two projective lines.

Answer

The line incident on

 

can be found from this determinant equation.

 

The equation for the point incident on two lines is the same.

Problem 4

Prove that the definition of incidence is independent of the choice of the representatives of   and  . That is, if  ,  ,  , and  ,  ,   are two triples of homogeneous coordinates for  , and  ,  ,  , and  ,  ,   are two triples of homogeneous coordinates for  , prove that   if and only if  .

Answer

If  ,  ,  , and  ,  ,   are two triples of homogeneous coordinates for   then the two column vectors are in proportion, that is, lie on the same line through the origin. Similarly, the two row vectors are in proportion.

 

Then multiplying gives the answer  .

Problem 5

Give a drawing to show that central projection does not preserve circles, that a circle may project to an ellipse. Can a (non-circular) ellipse project to a circle?

Answer

The picture of the solar eclipse — unless the image plane is exactly perpendicular to the line from the sun through the pinhole — shows the circle of the sun projecting to an image that is an ellipse. (Another example is that in many pictures in this Topic, the circle that is the sphere's equator is drawn as an ellipse, that is, is seen by a viewer of the drawing as an ellipse.)

The solar eclipse picture also shows the converse. If we picture the projection as going from left to right through the pinhole then the ellipse   projects through   to a circle  .

Problem 6

Give the formula for the correspondence between the non-equatorial part of the antipodal modal of the projective plane, and the plane  .

Answer

A spot on the unit sphere

 

is non-equatorial if and only if  . In that case it corresponds to this point on the   plane

 

since that is intersection of the line containing the vector and the plane.

Problem 7

(Pappus's Theorem) Assume that  ,  , and   are collinear and that  ,  , and   are collinear. Consider these three points: (i) the intersection   of the lines   and  , (ii) the intersection   of the lines   and  , and (iii) the intersection   of   and  .

  1. Draw a (Euclidean) picture.
  2. Apply the lemma used in Desargue's Theorem to get simple homogeneous coordinate vectors for the  's and  .
  3. Find the resulting homogeneous coordinate vectors for  's (these must each involve a parameter as, e.g.,   could be anywhere on the   line).
  4. Find the resulting homogeneous coordinate vectors for  . (Hint: it involves two parameters.)
  5. Find the resulting homogeneous coordinate vectors for  . (It also involves two parameters.)
  6. Show that the product of the three parameters is  .
  7. Verify that   is on the   line..
Answer
  1. Other pictures are possible, but this is one.

     

    The intersections  ,  , and   are labeled so that on each line is a  , a  , and a  .

  2. The lemma used in Desargue's Theorem gives a basis   with respect to which the points have these homogeneous coordinate vectors.
     
  3. First, any   on  
     
    has homogeneous coordinate vectors of this form
     
    (  is a parameter; it depends on where on the   line the point   is, but any point on that line has a homogeneous coordinate vector of this form for some  ). Similarly,   is on  
     
    and so has this homogeneous coordinate vector.
     
    Also similarly,   is incident on  
     
    and has this homogeneous coordinate vector.
     
  4. Because   is   we have this.
     
    Substituting   for   in the first equation
     
    shows that   has this two-parameter homogeneous coordinate vector.
     
  5. Since   is the intersection  
     
    and substituting   for   in the first equation
     
    gives that   has this two-parameter homogeneous coordinate vector.
     
  6. Because   is on the   line its homogeneous coordinate vector has the form
     
    but a previous part of this question established that  's homogeneous coordinate vectors have the form
     
    and so this a homogeneous coordinate vector for  .
     
    By ( ) and ( ), there is a relationship among the three parameters:  .
  7. The homogeneous coordinate vector of   can be written in this way.
     
    Now, the   line consists of the points whose homogeneous coordinates have this form.
     
    Taking   and   shows that the homogeneous coordinate vectors of   have this form.