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