Perhaps you should try some spelling variations:
20 instability » 2 instability, 10 instability, 26 instability
2c instabilit » sc instability, 2.2 instability, 2.4 instability
6 instability » _ instability, 7 instability, 5 instability
4 instability » _ instability, 7 instability, 5 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 (((((2((6 instability) OR (4 instability))) OR (1((6 instability) OR (4 instability))))) OR (((6 instability) OR (4 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 (((((2((6 instability) OR (4 instability))) or (1((6 instability) OR (4 instability))))) or (((6 instability) OR (4 instability))))))))%c3%a4t %2c lehrbuch%22".
• Adding a wildcard symbol may retrieve word variants: %22str%c3%b6mungsmechanik %((2c instabilit) or (((20 instability) or (((((2((6 instability) OR (4 instability))) or (1((6 instability) OR (4 instability))))) or (((6 instability) OR (4 instability))))))))%c3%a4t %2c lehrbuch%22*.