Perhaps you should try some spelling variations:
disc instability » disk instability, disks instability, sc instability
disc stability » disk stability, disks stability, dsgc stability
20 instability » 2 instability, 10 instability, 26 instability
6 instability » _ instability, 7 instability, 5 instability
zu lehrbuch » 1 lehrbuch, 2 lehrbuch, 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 %((((disc instability) OR (disc stability))) OR (((20 instability) OR (6 instability))))%c3%a4t %zu lehrbuch%22.
• The words AND, OR and NOT may confuse the search; try adding quotes: "%22str%c3%b6mungsmechanik %((((disc instability) OR (disc stability))) or (((20 instability) or (6 instability))))%c3%a4t %zu lehrbuch%22".
• Adding a wildcard symbol may retrieve word variants: %22str%c3%b6mungsmechanik %((((disc instability) OR (disc stability))) or (((20 instability) or (6 instability))))%c3%a4t %zu lehrbuch%22*.