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