Perhaps you should try some spelling variations:
2.2 instability » 7.2 instability, 6.2 instability, 2.4 instability
26 instability » 2.6 instability, 16 instability, 2.4 instability
20 instability » 10 instability, 2.4 instability, 2.6 instability
27 instability » 7 instability, 17 instability, 37 instability
6 instability » _ instability, 7 instability, 5 instability
2c instabilit » sc instability, 2.4 instability, 2.6 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 (2.2 instability))) OR (27 instability))) OR (((26 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 (2.2 instability))) or (27 instability))) or (((26 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 (2.2 instability))) or (27 instability))) or (((26 instability) or (6 instability))))))%c3%a4t %((2c lehrbuch) OR (2 lehrbuch))%22*.