Perhaps you should try some spelling variations:
10 instability » 1 instability, 17 instability, _ instability
2c instabilit » 20 instability, sc instability, 2.2 instability
7 instability » _ instability
2c lehrbuch » 1 lehrbuch, zu lehrbuch, im lehrbuch
2 lehrbuch » 1 lehrbuch, 2 lehrbucher, zu 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 (((10 instability) OR (7 instability))))%c3%a4t %((2c lehrbuch) OR (2 lehrbuch))%22.
• The words AND, OR and NOT may confuse the search; try adding quotes: "%22str%c3%b6mungsmechanik %((2c instabilit) or (((10 instability) or (7 instability))))%c3%a4t %((2c lehrbuch) OR (2 lehrbuch))%22".
• Adding a wildcard symbol may retrieve word variants: %22str%c3%b6mungsmechanik %((2c instabilit) or (((10 instability) or (7 instability))))%c3%a4t %((2c lehrbuch) OR (2 lehrbuch))%22*.