Perhaps you should try some spelling variations:
2 instability » _ instability, 7 instability, 5 instability
2 lehrbucher » 1 lehrbucher, a lehrbucher, _ lehrbuchern
im lehrbuch » zum lehrbuch, ihk lehrbuch, dem lehrbuch
2 lehrbuch » 1 lehrbuch, und lehrbuch, das lehrbuch
ein » eine

Try widening your search to All Fields.

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