Perhaps you should try some spelling variations:
2.4 instability » 2.6 instability, 5.2.4 instability, 2.4 stability
3.4 instability » 3.5 instability, 3.4 stability, 3 instability
2.2 instability » 7.2 instability, 6.2 instability, 2.6 instability
27 instability » 7 instability, 17 instability, 20 instability
26 instability » 2.6 instability, 6 instability, 16 instability
2c instabilit » 20 instability, sc instability, 2.6 instability
zu lehrbuch » 1 lehrbuch, 2 lehrbuch, 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 (((2.4 instability) OR (3.4 instability))))) OR (2.2 instability))) OR (((27 instability) OR (26 instability))))%c3%a4t %zu 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 (3.4 instability))))) or (2.2 instability))) or (((27 instability) or (26 instability))))%c3%a4t %zu lehrbuch%22".
• Adding a wildcard symbol may retrieve word variants: %22str%c3%b6mungsmechanik %((((((2c instabilit) or (((2.4 instability) OR (3.4 instability))))) or (2.2 instability))) or (((27 instability) or (26 instability))))%c3%a4t %zu lehrbuch%22*.