Perhaps you should try some spelling variations:
"racial differences equations" » "racial difference equation", "racial differential equations", "racial differentes situations"
"regional differences equations" » "regional difference equation", "regional differential equations", "regional differentes situations"
"potential difference equations" » "potential differential 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 ("regional differences equations"))) OR ("potential difference equations")).
• The words AND, OR and NOT may confuse the search; try adding quotes: "((((\"racial differences equations\") OR (\"regional differences equations\"))) or (\"potential difference equations\"))".
• Removing quotes may allow a broader search: (((( racial differences equations ) OR ( regional differences equations ))) or ( potential difference equations )).
• Adding a wildcard symbol may retrieve word variants: (((("racial differences equations") OR ("regional differences equations"))) or ("potential difference equations"))*.