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