Perhaps you should try some spelling variations:
26 instability » 2.6 instability, 2 instability, 6 instability
10 instability » 17 instability, _ instability, 7 instability
20 instability » 2 instability, 27 instability, 2.2 instability
2c instabilit » sc instability, 2.2 instability, 2.4 instability
1 instability » _ instability, 7 instability, 5 instability
2c lehrbuch » 2 lehrbuch, 1 lehrbuch, im lehrbuch
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 (26 instability))))) OR (((10 instability) OR (1 instability))))%c3%a4t %((2c lehrbuch) OR (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 (26 instability))))) or (((10 instability) or (1 instability))))%c3%a4t %((2c lehrbuch) OR (zu lehrbuch))%22".
• Adding a wildcard symbol may retrieve word variants: %22str%c3%b6mungsmechanik %((((2c instabilit) or (((20 instability) or (26 instability))))) or (((10 instability) or (1 instability))))%c3%a4t %((2c lehrbuch) OR (zu lehrbuch))%22*.