Perhaps you should try some spelling variations:
sc instability » disc instability, 7 instability, 2 instability
10 instability » 17 instability, _ instability, 7 instability
2c instabilit » 20 instability, 2.2 instability, 2.4 instability
4 instability » _ instability, 7 instability
5 instability » _ instability, 7 instability
1 instability » _ instability, 7 instability, 2 instability
2c lehrbuch » 2 lehrbuch, 1 lehrbuch, im 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 (4 instability))))) OR (((((10 instability) OR (5 instability))) OR (1 instability))))%c3%a4t %2c lehrbuch%22.
• The words AND, OR and NOT may confuse the search; try adding quotes: "%22str%c3%b6mungsmechanik %((((2c instabilit) or (((sc instability) OR (4 instability))))) or (((((10 instability) or (5 instability))) or (1 instability))))%c3%a4t %2c lehrbuch%22".
• Adding a wildcard symbol may retrieve word variants: %22str%c3%b6mungsmechanik %((((2c instabilit) or (((sc instability) OR (4 instability))))) or (((((10 instability) or (5 instability))) or (1 instability))))%c3%a4t %2c lehrbuch%22*.