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