Perhaps you should try some spelling variations:
sc instability » disc instability, 7 instability, 4 instability
20 instability » 2 instability, 10 instability, 26 instability
2c instabilit » 2.2 instability, 2.4 instability, 2.6 instability
5 instability » _ instability, 7 instability
2c lehrbuch » 1 lehrbuch, zu lehrbuch, es lehrbuch
im lehrbuch » ein lehrbuch, zum lehrbuch, ihk lehrbuch
2 lehrbuch » 1 lehrbuch, 2 lehrbucher, zu lehrbuch

You may be able to get more results by adjusting your search query.

• If you are trying to use Boolean operators, they must be ALL CAPS: %22str%c3%b6mungsmechanik %((((2c instabilit) OR (((sc instability) OR (5 instability))))) OR (20 instability))%c3%a4t %((((2c lehrbuch) OR (im lehrbuch))) OR (2 lehrbuch))%22.
• The words AND, OR and NOT may confuse the search; try adding quotes: "%22str%c3%b6mungsmechanik %((((2c instabilit) or (((sc instability) OR (5 instability))))) or (20 instability))%c3%a4t %((((2c lehrbuch) or (im lehrbuch))) or (2 lehrbuch))%22".
• Adding a wildcard symbol may retrieve word variants: %22str%c3%b6mungsmechanik %((((2c instabilit) or (((sc instability) OR (5 instability))))) or (20 instability))%c3%a4t %((((2c lehrbuch) or (im lehrbuch))) or (2 lehrbuch))%22*.