Exercise 5 I just tidied up the proof a bit 1st April 2015
Exercise 11 Added some more explanation on 24 June 2015
Exercise 14 slight change of notation to avoid confusion 15/07/15
Exercise 17 Made figure 2 consistent with figure 1. 14/08/15
Exercise 18 Added some vertical bars I had forgotten in 2 places 28/4/15
Exercise 19 Corrected a couple of typos 6/9/2015
Exercise 20 All three parts now. 10/Sept/2015. Corrected 25/9/2015 (see blog)