Perhaps you should try some spelling variations:
"nonlinear potential theory" » "nonlinear partial theory", "nonlinear material theory", "nonlinear properties theory"
"nonlinear particle theory" » "nonlinear partial theory", "collinear particles theory", "linear particle theory"

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: (("nonlinear potential theory") OR ("nonlinear particle theory")).
• Removing quotes may allow a broader search: (( nonlinear potential theory ) or ( nonlinear particle theory )).
• Adding a wildcard symbol may retrieve word variants: (("nonlinear potential theory") or ("nonlinear particle theory"))*.