Perhaps you should try some spelling variations:
2 instability » _ instability, 7 instability, 1 instability
5 instability » _ instability, 7 instability
4 instability » _ instability, 7 instability
2c lehrbuch » 2 lehrbuch, 1 lehrbuch, es lehrbuch
im lehrbuch » zum lehrbuch, ihk lehrbuch, dem 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) OR (5 instability))) OR (4 instability))%c3%a4t %((2c lehrbuch) 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) OR (5 instability))) or (4 instability))%c3%a4t %((2c lehrbuch) or (((im lehrbuch) or (ein lehrbuch))))%22".
• Adding a wildcard symbol may retrieve word variants: %22str%c3%b6mungsmechanik %((((2 instability) OR (5 instability))) or (4 instability))%c3%a4t %((2c lehrbuch) or (((im lehrbuch) or (ein lehrbuch))))%22*.