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, und lehrbuch, das lehrbuch
2c lehrbuch » im lehrbuch, es lehrbuch, i lehrbuch
1 lehrbuch » ein lehrbuch, das lehrbuch
2 lehrbuch » 2 lehrbucher, ein 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 (20 instability))%c3%a4t %((((((2c lehrbuch) OR (1 lehrbuch))) OR (ihk lehrbuch))) OR (2 lehrbuch))%22.
• The words AND, OR and NOT may confuse the search; try adding quotes: "%22str%c3%b6mungsmechanik %((2c instabilit) or (20 instability))%c3%a4t %((((((2c lehrbuch) OR (1 lehrbuch))) or (ihk lehrbuch))) or (2 lehrbuch))%22".
• Adding a wildcard symbol may retrieve word variants: %22str%c3%b6mungsmechanik %((2c instabilit) or (20 instability))%c3%a4t %((((((2c lehrbuch) OR (1 lehrbuch))) or (ihk lehrbuch))) or (2 lehrbuch))%22*.