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
ihk lehrbuch » ein lehrbuch, das lehrbuch
und lehrbuch » ein lehrbuch
2c lehrbuch » 2 lehrbuch, im lehrbuch, es lehrbuch
1 lehrbuch » ein lehrbuch, das lehrbuch, dem 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))%c3%a4t %((((2c lehrbuch) OR (((ihk lehrbuch) OR (und lehrbuch))))) OR (1 lehrbuch))%22.
• Adding a wildcard symbol may retrieve word variants: %22str%c3%b6mungsmechanik %((2c instabilit) or (20 instability))%c3%a4t %((((2c lehrbuch) or (((ihk lehrbuch) or (und lehrbuch))))) or (1 lehrbuch))%22*.