Perhaps you should try some spelling variations:
2.2 instability » 7.2 instability, 6.2 instability, 2.4 instability
20 instability » 10 instability, 27 instability, 2.4 instability
26 instability » 2.6 instability, 6 instability, 16 instability
2c instabilit » sc instability, 2.4 instability, 2.6 instability
zu lehrbuch » 1 lehrbuch, ein lehrbuch, und 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 (2.2 instability))) OR (((20 instability) OR (26 instability))))%c3%a4t %((zu lehrbuch) OR (2 lehrbuch))%22.
• The words AND, OR and NOT may confuse the search; try adding quotes: "%22str%c3%b6mungsmechanik %((((2c instabilit) OR (2.2 instability))) or (((20 instability) or (26 instability))))%c3%a4t %((zu lehrbuch) or (2 lehrbuch))%22".
• Adding a wildcard symbol may retrieve word variants: %22str%c3%b6mungsmechanik %((((2c instabilit) OR (2.2 instability))) or (((20 instability) or (26 instability))))%c3%a4t %((zu lehrbuch) or (2 lehrbuch))%22*.