put equation into split
realign and add labels
make use of references
shorten equation
complement text
a lot of changes
enhance readability
update mathcals and some little tweaks
add fifth lecture

combination will be done later, this ensures that we don't lose any
text while combining the files
FIX extra $
new mathcals
new textnormal commands
satz numeration will orient at section, like the prof in the lecture
remove unnecessary space