Perhaps you should try some spelling variations:
reef history » brief history, reeves history, reef fishery
20th century » 18th century
ref history » brief history, bief history, imf history
2020 20th » 2020 16th, 2020 2021, 2020 5th

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 %((7 germany) OR (1917 german)) %((ref history) OR (reef history)) %2020 20th century%22.
• The words AND, OR and NOT may confuse the search; try adding quotes: "%22agriculture and state %((7 germany) or (1917 german)) %((ref history) OR (reef history)) %2020 20th century%22".
• Adding a wildcard symbol may retrieve word variants: %22agriculture and state %((7 germany) or (1917 german)) %((ref history) OR (reef history)) %2020 20th century%22*.