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