Course MT4521 Geometry and topology

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## Exercises 8

1. Let f be the element f = (ax + b)/(cx - a) with a, bR, a2 + bc > 0. Find the order of f in the projective group PGL(2, R).
Let g be the element g = (x -1)/x and let h be h = 1 - x. Find the orders of g and h in PGL(2, R).
Calculate the elements of the subgroup of PGL(2, R) generated by g and h. What is the group?

2. Show that any three distinct points on a projective line can be mapped to any three distinct points on a second line by positioning the lines suitably in a plane and projecting from a point not on either line.

3. Show that any four points (no three on a line) in RP2 can be mapped to any other four such points by a projective transformation.
[Hint: Show first that one can map [α, 0, 0], [0, β, 0] and [0, 0, γ] to three of the points. Then choose α, β, γ so that [1, 1, 1] is mapped to the fourth point.]

4. Given any vector space V over a field F, we can form its associated projective space P(V) by using the construction from lectures:
P(V) = V - {0}/~ where ~ is the equivalence relation u ~ v if u = λv for u, vV - {0} and λF.
Let F be a finite field with pk elements where p is a prime number.
Prove that a projective line over F has pk + 1 points on it and that in general if V has dimension n over F, then |P(V)| = (pkn - 1)/(pk - 1). Deduce that if |F| = 2 and n = 3 we get a projective plane with 7 points.
(The picture on the right represents such a plane. It has 7 points and 7 lines: each point lies on 3 lines and each line contains 3 points. One of the lines is represented by a circle.)

Describe how one could construct projective planes with 13 points and with 21 points.

Solution to question 4 