Frequently asked questions
Question: In Section 10.3, problem 6 uses problem 5 which we did not do.
Any hint?

Answer: Here is the solution to problem 5:

Question: What exactly is this Jordan Normal form?

Answer: Izzet has written down
something about it.

Question: Why don't we just use the formula P=AA^{T} to find the matrix P
of the orthogonal projection onto a subspace V?

Answer: the formula P=A A^{T} gives the orthogonal projection
if the column vectors of A are orthonormal vectors only. In general
one has to use a formula which was derived in least square solutions:
P= A (A^{T} A)^{1} A^{T} which simplifies in the case of
orthonormal columns to A A^{T} because the middle part is then an
identity matrix.

Question: Is there any difference between a space and a linear space
and between a linear space and a subspace, and between a
space and a subspace.

Answer:
Sometimes, space is used as an abbreviation for the three
dimensional linear space. If a linear space is a subset of
an other linear space, one calls it a "linear subspace".
But it is still a linear space.
Since any linear space is also a subspace of itself,
it is also a "linear subspace". The additional "sub" can
be used to stress that something is contained in something
else.

Question: How come that two rotations don't commute, but two
rotationdilations do?

Answer:
The key is the dimension. Rotations in the plane commute,
while rotations in space do not.
Also, two rotationdilations commute in the plane,
while they do not commute in space.

Question:
Are starred, "recommended" problems meant to be optional enrichment
exercises or exercises to be turned in with the assignment? I was
planning on doing them on my own, but not turning them in with the
assignment.

Answer:
Recommended problems don't have to be turned in and CA's don't
have to grade them. We plan to post solutions to the recommended
problems and recommend to solve those problems if time permits.
Otherwise, focus on the main problems. Don't feel pressured to do
them. You can get perfect scores without doing them. You can use
them also as practice problems for the exams.

Question: When do classes start and what book will we used?

Answer: Check out the syllabus page.

Question: When will I get my section assignment?

Answer: You should be notified by the program latest until Friday night.
If you should not have got the section assignment until then, contact the course head
Oliver Knill (knill@math).

