Perhaps you should try some spelling variations:
2.4 instability » 3.4 instability, 2.2 instability, 2.6 instability
20 instability » 2 instability, 27 instability, 2.2 instability
26 instability » 2.6 instability, 2 instability, 6 instability
10 instability » 1 instability, 17 instability, _ instability
2c instabilit » sc instability, 2.2 instability, 2.6 instability
zu lehrbuch » 2 lehrbuch, ein lehrbuch, und lehrbuch
1 lehrbuch » ein lehrbuch, das lehrbuch, ihk 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 (2.4 instability))) OR (((((20 instability) OR (10 instability))) OR (26 instability))))%c3%a4t %((zu lehrbuch) OR (1 lehrbuch))%22.
• The words AND, OR and NOT may confuse the search; try adding quotes: "%22str%c3%b6mungsmechanik %((((2c instabilit) or (2.4 instability))) or (((((20 instability) or (10 instability))) or (26 instability))))%c3%a4t %((zu lehrbuch) OR (1 lehrbuch))%22".
• Adding a wildcard symbol may retrieve word variants: %22str%c3%b6mungsmechanik %((((2c instabilit) or (2.4 instability))) or (((((20 instability) or (10 instability))) or (26 instability))))%c3%a4t %((zu lehrbuch) OR (1 lehrbuch))%22*.