Perhaps you should try some spelling variations:
"mark's theory" » "markets theory", "marty's theory", "market theory"
"marr's theory" » "marty's theory", "mayer's theory", "marcus theory"
"marx's theory" » "marc's theory", "mary's theory", "max's theory"

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: (("marx's theory") OR ((("mark's theory") OR ("marr's theory")))).
• The words AND, OR and NOT may confuse the search; try adding quotes: "((\"marx's theory\") or (((\"mark's theory\") OR (\"marr's theory\"))))".
• Removing quotes may allow a broader search: (( marx's theory ) or ((( mark's theory ) OR ( marr's theory )))).
• Adding a wildcard symbol may retrieve word variants: (("marx's theory") or ((("mark's theory") OR ("marr's theory"))))*.