Perhaps you should try some spelling variations:
10 instability » 1 instability, 17 instability, _ instability
27 instability » 2 instability, 17 instability, 20 instability
2c instabilit » 20 instability, sc instability, 2 instability
7 instability » _ 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 (((((10 instability) OR (27 instability))) OR (((27 instability) OR (7 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 (((((10 instability) or (27 instability))) or (((27 instability) or (7 instability))))))%c3%a4t %((2c lehrbuch) OR (2 lehrbuch))%22".
• Adding a wildcard symbol may retrieve word variants: %22str%c3%b6mungsmechanik %((2c instabilit) or (((((10 instability) or (27 instability))) or (((27 instability) or (7 instability))))))%c3%a4t %((2c lehrbuch) OR (2 lehrbuch))%22*.