Perhaps you should try some spelling variations:
"nonlinear potential theory" » "nonlinear partial theory", "nonlinear material theory", "nonlinear properties theory"
"nonlinear spatial theory" » "nonlinear partial theory", "nonlinear spectral theory", "nonlinear static theory", "nonlinear special theory", "nonlinear social theory", "nonlinear spatial memory"

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