Perhaps you should try some spelling variations:
disc instability » disk instability, disks instability, disc stability
20 instability » 10 instability, 26 instability, 27 instability
2 instability » _ instability, 7 instability, 5 instability
zu lehrbuch » 2 lehrbuch, ein lehrbuch, und lehrbuch
1 lehrbuch » ein lehrbuch, das lehrbuch, ihk 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 %((disc instability) OR (((20 instability) OR (2 instability))))%c3%a4t %((zu lehrbuch) OR (1 lehrbuch))%22.
• The words AND, OR and NOT may confuse the search; try adding quotes: "%22str%c3%b6mungsmechanik %((disc instability) or (((20 instability) or (2 instability))))%c3%a4t %((zu lehrbuch) OR (1 lehrbuch))%22".
• Adding a wildcard symbol may retrieve word variants: %22str%c3%b6mungsmechanik %((disc instability) or (((20 instability) or (2 instability))))%c3%a4t %((zu lehrbuch) OR (1 lehrbuch))%22*.