Perhaps you should try some spelling variations:
20 instability » 2 instability, 10 instability, 27 instability
26 instability » 2.6 instability, 2 instability, 6 instability
2c instabilit » sc instability, 2.2 instability, 2.4 instability
2c lehrbuch » 1 lehrbuch, es lehrbuch, i lehrbuch
im lehrbuch » ein lehrbuch, zum lehrbuch, ihk 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 %((2c instabilit) OR (((20 instability) OR (26 instability))))%c3%a4t %((((2c lehrbuch) OR (im 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) OR (26 instability))))%c3%a4t %((((2c lehrbuch) or (im lehrbuch))) or (2 lehrbuch))%22".
• Adding a wildcard symbol may retrieve word variants: %22str%c3%b6mungsmechanik %((2c instabilit) or (((20 instability) OR (26 instability))))%c3%a4t %((((2c lehrbuch) or (im lehrbuch))) or (2 lehrbuch))%22*.