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