Perhaps you should try some spelling variations:
20 instability » 10 instability, 26 instability, 27 instability
2c instabilit » sc instability, 2.2 instability, 2.4 instability
2 instability » _ instability, 7 instability, 5 instability
ihk lehrbuch » ein lehrbuch, zum lehrbuch, und lehrbuch
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 (((20 instability) OR (2 instability))))%c3%a4t %((((2c 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) OR (2 instability))))%c3%a4t %((((2c lehrbuch) or (ihk lehrbuch))) or (2 lehrbuch))%22".
• Adding a wildcard symbol may retrieve word variants: %22str%c3%b6mungsmechanik %((2c instabilit) or (((20 instability) OR (2 instability))))%c3%a4t %((((2c lehrbuch) or (ihk lehrbuch))) or (2 lehrbuch))%22*.