Perhaps you should try some spelling variations:
27 instability » 7 instability, 17 instability, 37 instability
26 instability » 2.6 instability, 16 instability, 2.2 instability
20 instability » 10 instability, 2.2 instability, 2.4 instability
2c instabilit » sc instability, 2.2 instability, 2.4 instability
2 instability » _ instability, 7 instability, 5 instability
6 instability » _ instability, 7 instability, 5 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 (((((26 instability) OR (2 instability))) OR (6 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 (((((26 instability) OR (2 instability))) or (6 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 (((((26 instability) OR (2 instability))) or (6 instability))))))%c3%a4t %((2c lehrbuch) or (2 lehrbuch))%22*.