Perhaps you should try some spelling variations:
"racial differences equations" » "racial difference equations", "racial difference equation", "racial differential equations"
"regional differences equations" » "regional difference equations", "regional difference equation", "regional differential equations"

You may be able to get more results by adjusting your search query.

• The words AND, OR and NOT may confuse the search; try adding quotes: "((\"racial differences equations\") OR (\"regional differences equations\"))*".
• Removing quotes may allow a broader search: (( racial differences equations ) OR ( regional differences equations ))*.