Perhaps you should try some spelling variations:
20 instability » 2 instability, 10 instability, 26 instability
2c instabilit » sc instability, 2.2 instability, 2.4 instability
2 lehrbucher » 1 lehrbucher, a lehrbucher, _ lehrbuchern
2c lehrbuch » 1 lehrbuch, es lehrbuch, i lehrbuch
im lehrbuch » ein lehrbuch, zum lehrbuch, ihk lehrbuch
2 lehrbuch » 1 lehrbuch, ein lehrbuch, und lehrbuch

You may be able to get more results by adjusting your search query.

• The words AND, OR and NOT may confuse the search; try adding quotes: "%22str%c3%b6mungsmechanik %((2c instabilit) OR (20 instability))%c3%a4t %((((2c lehrbuch) OR (im lehrbuch))) OR (((2 lehrbuch) OR (2 lehrbucher))))%22".
• Adding a wildcard symbol may retrieve word variants: %22str%c3%b6mungsmechanik %((2c instabilit) OR (20 instability))%c3%a4t %((((2c lehrbuch) OR (im lehrbuch))) OR (((2 lehrbuch) OR (2 lehrbucher))))%22*.