Perhaps you should try some spelling variations:
"racial differences equations" » "racial difference equation", "racial differential equations", "racial differentes situations", "regional differences equations"
"national differences equations" » "national difference equation", "national differential equations", "national differentes situations", "functional differences equations", "regional differences equations", "generational differences equations"
"regional difference equations" » "regional differential equations", "regional differences equations", "regional conference equations", "regional reference equations"

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: (((("racial differences equations") OR ("national differences equations"))) OR ("regional difference equations")).
• The words AND, OR and NOT may confuse the search; try adding quotes: "((((\"racial differences equations\") OR (\"national differences equations\"))) or (\"regional difference equations\"))".
• Removing quotes may allow a broader search: (((( racial differences equations ) OR ( national differences equations ))) or ( regional difference equations )).
• Adding a wildcard symbol may retrieve word variants: (((("racial differences equations") OR ("national differences equations"))) or ("regional difference equations"))*.