Perhaps you should try some spelling variations:
2.4 instability » 3.4 instability, 2.2 instability, 2.6 instability
20 instability » 2 instability, 26 instability, 27 instability
10 instability » 1 instability, 17 instability, _ instability
2c lehrbuch » 1 lehrbuch, es lehrbuch, i lehrbuch
im lehrbuch » ein lehrbuch, zum lehrbuch, ihk lehrbuch
zu lehrbuch » 1 lehrbuch, ein lehrbuch, und lehrbuch
2 lehrbuch » 1 lehrbuch, 2 lehrbucher, ein 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) OR (10 instability))))%c3%a4t %((((2c lehrbuch) OR (im lehrbuch))) OR (((2 lehrbuch) OR (zu lehrbuch))))%22.
• The words AND, OR and NOT may confuse the search; try adding quotes: "%22str%c3%b6mungsmechanik %((2.4 instability) or (((20 instability) or (10 instability))))%c3%a4t %((((2c lehrbuch) or (im lehrbuch))) or (((2 lehrbuch) OR (zu lehrbuch))))%22".
• Adding a wildcard symbol may retrieve word variants: %22str%c3%b6mungsmechanik %((2.4 instability) or (((20 instability) or (10 instability))))%c3%a4t %((((2c lehrbuch) or (im lehrbuch))) or (((2 lehrbuch) OR (zu lehrbuch))))%22*.