These pages might be of use for students, especially those who take a course with me or whom I supervise in an assignment or internship. I address some issues here that I have to explain quite often. Don't get me wrong, I like explaining - hence these pages. But as there isn't always enough time to explain this in class, you might help yourself here. Mind that these are not authoritative sources. That is, you may not cite them as you would normally do with (academic) literature. Rather, you may presume these things as background knowledge without further reference (unless of course they are themselves central to the problem you address; but in those cases, these texts will be too superficial anyway!). You don't even have to thank me, but neither is it forbidden.