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