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