Perhaps you should try some spelling variations:
2.4 instability » 3.4 instability, 2.2 instability, 2.6 instability
20 instability » 2 instability, 10 instability, 26 instability
2c lehrbuch » es lehrbuch, i lehrbuch, ein lehrbuch
im lehrbuch » ein lehrbuch, zum lehrbuch, ihk lehrbuch
2 lehrbuch » 2 lehrbucher, ein lehrbuch, das lehrbuch
1 lehrbuch » ein lehrbuch, das lehrbuch, ihk 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 %((2.4 instability) OR (20 instability))%c3%a4t %((((2c lehrbuch) OR (im lehrbuch))) OR (((2 lehrbuch) OR (1 lehrbuch))))%22.
• The words AND, OR and NOT may confuse the search; try adding quotes: "%22str%c3%b6mungsmechanik %((2.4 instability) or (20 instability))%c3%a4t %((((2c lehrbuch) or (im lehrbuch))) or (((2 lehrbuch) OR (1 lehrbuch))))%22".
• Adding a wildcard symbol may retrieve word variants: %22str%c3%b6mungsmechanik %((2.4 instability) or (20 instability))%c3%a4t %((((2c lehrbuch) or (im lehrbuch))) or (((2 lehrbuch) OR (1 lehrbuch))))%22*.