Wrong "knots" in exercise 3.12.

A student has made me aware (via Ulrik) that I had misinterpreted exercise 3.12. In particular, we were supposed to glue the piecewise polynomials together at the "interior knots" and not at the "knots" in this exercise. The idea of the exercise remains the same, but the expressions become slightly different, and they agree with what can be found in the book. I've posted a new solution to this exercise under "Teaching sessions" now. Hopefully, everything is correct now, but let me know if you find typos. This exercise contains a lot of indices. 

Best,
Vegard