Perhaps you should try some spelling variations:
20 instability » 2 instability, 10 instability, 26 instability
27 instability » 7 instability, 2 instability, 17 instability
25 instability » 2 instability, 15 instability, 26 instability
2c instabilit » sc instability, 2 instability, 2.2 instability
5 instability » _ instability, 7 instability
2c lehrbuch » 1 lehrbuch, im lehrbuch, es lehrbuch
2 lehrbuch » 1 lehrbuch, 2 lehrbucher, ein 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 (((((20 instability) OR (27 instability))) OR (((25 instability) OR (5 instability))))))%c3%a4t %((2c lehrbuch) OR (2 lehrbuch))%22.
• The words AND, OR and NOT may confuse the search; try adding quotes: "%22str%c3%b6mungsmechanik %((2c instabilit) or (((((20 instability) or (27 instability))) or (((25 instability) or (5 instability))))))%c3%a4t %((2c lehrbuch) OR (2 lehrbuch))%22".
• Adding a wildcard symbol may retrieve word variants: %22str%c3%b6mungsmechanik %((2c instabilit) or (((((20 instability) or (27 instability))) or (((25 instability) or (5 instability))))))%c3%a4t %((2c lehrbuch) OR (2 lehrbuch))%22*.