Perhaps you should try some spelling variations:
"racial differences equations" » "racial difference equations", "racial difference equation", "racial differential equations", "regional differences equations"
"national differences equations" » "national difference equations", "national difference equation", "national differential equations", "functional differences equations", "regional differences equations", "generational differences 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 (\"national differences equations\"))".
• Removing quotes may allow a broader search: (( racial differences equations ) OR ( national differences equations )).
• Adding a wildcard symbol may retrieve word variants: (("racial differences equations") OR ("national differences equations"))*.