Perhaps you should try some spelling variations:
"regional differential equation" » "regional differentials equation", "original differential equation", "regional preferential equation", "regional differential equations"
"racial difference equations" » "racial differential equations", "racial differences equations", "facial difference equations", "social 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: (("racial difference equations") OR ("regional differential equation"))*.
• Removing quotes may allow a broader search: (( racial difference equations ) or ( regional differential equation ))*.