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