Perhaps you should try some spelling variations:
"nonlinear potential theory" » "nonlinear partial theory", "nonlinear properties theory"
"nonlinear material theory" » "nonlinear partial theory", "nonlinear media theory", "nonlinear water theory"
"nonlinear materials theory" » "nonlinear partials theory", "nonlinear medias theory", "nonlinear waters theory"
"nonlinear matrix theory" » "linear matrix theory", "nonlinear media theory", "nonlinear multi 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 material theory") OR ("nonlinear matrix theory"))) OR ("nonlinear materials theory")))).
• The words AND, OR and NOT may confuse the search; try adding quotes: "((\"nonlinear potential theory\") or (((((\"nonlinear material theory\") OR (\"nonlinear matrix theory\"))) or (\"nonlinear materials theory\"))))".
• Removing quotes may allow a broader search: (( nonlinear potential theory ) or ((((( nonlinear material theory ) OR ( nonlinear matrix theory ))) or ( nonlinear materials theory )))).
• Adding a wildcard symbol may retrieve word variants: (("nonlinear potential theory") or ((((("nonlinear material theory") OR ("nonlinear matrix theory"))) or ("nonlinear materials theory"))))*.