Perhaps you should try some spelling variations:
regional differential » regional differentials, original differential, regional preferential
difference equation » difference equations, differential equation
racial difference » racial differences, facial difference, social difference

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: (( regional differential equations ) OR ( racial difference equation )).
• Adding a wildcard symbol may retrieve word variants: (( regional differential equations ) or ( racial difference equation ))*.