Perhaps you should try some spelling variations:
20 instability » 2 instability, 26 instability, 27 instability
10 instability » 1 instability, 7 instability, 5 instability
17 instability » 7 instability, 1 instability, 5 instability
2c instabilit » sc instability, 2.2 instability, 2.4 instability
2c lehrbuch » 2 lehrbuch, 1 lehrbuch, im 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 (((10 instability) OR (17 instability))))%c3%a4t %2c lehrbuch%22.
• The words AND, OR and NOT may confuse the search; try adding quotes: "%22str%c3%b6mungsmechanik %((((2c instabilit) OR (20 instability))) or (((10 instability) or (17 instability))))%c3%a4t %2c lehrbuch%22".
• Adding a wildcard symbol may retrieve word variants: %22str%c3%b6mungsmechanik %((((2c instabilit) OR (20 instability))) or (((10 instability) or (17 instability))))%c3%a4t %2c lehrbuch%22*.