Perhaps you should try some spelling variations:
ref history » reef history, brief history, bief history
21 germany » 2.1 germany, 11 germany, 2 germany
2019 20th » 2019 4th, 2019 5th, 2019 16th
2019 6th » 2019 4th
2f 20th » 20 20th, 2.2 20th

Try widening your search to All Fields.

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: %22agriculture AND state %21 germany %ref history %((2f 20th) OR (((2019 20th) OR (2019 6th)))) century%22.
• The words AND, OR and NOT may confuse the search; try adding quotes: "%22agriculture and state %21 germany %ref history %((2f 20th) or (((2019 20th) OR (2019 6th)))) century%22".
• Adding a wildcard symbol may retrieve word variants: %22agriculture and state %21 germany %ref history %((2f 20th) or (((2019 20th) OR (2019 6th)))) century%22*.