Perhaps you should try some spelling variations:
26 instability » 2.6 instability, 2 instability, 6 instability
20 instability » 2 instability, 10 instability, 27 instability
2c instabilit » sc instability, 2 instability, 2.2 instability
2 lehrbucher » a lehrbucher, und lehrbucher, die lehrbucher
1 lehrbucher » a lehrbucher, 1 lehrbuch, und lehrbucher
zu lehrbuch » 1 lehrbuch, ein lehrbuch, und lehrbuch
2 lehrbuch » 1 lehrbuch, ein lehrbuch, und 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 %((zu lehrbuch) OR (((2 lehrbuch) OR (((((2 lehrbucher) OR (1 lehrbucher))) OR (_ lehrbuchern))))))%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 %((zu lehrbuch) or (((2 lehrbuch) or (((((2 lehrbucher) OR (1 lehrbucher))) or (_ lehrbuchern))))))%22".
• Adding a wildcard symbol may retrieve word variants: %22str%c3%b6mungsmechanik %((2c instabilit) or (((20 instability) or (26 instability))))%c3%a4t %((zu lehrbuch) or (((2 lehrbuch) or (((((2 lehrbucher) OR (1 lehrbucher))) or (_ lehrbuchern))))))%22*.