Perhaps you should try some spelling variations:
"nonlinear potential theory" » "nonlinear partial theory", "nonlinear material theory", "nonlinear properties theory"
"nonlinear social theory" » "nonlinear soil theory", "nonlinear spatial theory", "nonlinear solid 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 social theory")).
• Removing quotes may allow a broader search: (( nonlinear potential theory ) or ( nonlinear social theory )).
• Adding a wildcard symbol may retrieve word variants: (("nonlinear potential theory") or ("nonlinear social theory"))*.