Perhaps you should try some spelling variations:
"nonlinear material theory" » "nonlinear materials theory", "nonlinear matrix theory", "nonlinear partial theory"
"nonlinear spectral theory" » "linear spectral theory", "nonlinear structural theory", "nonlinear aspects 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 material theory") OR ("nonlinear spectral theory")).
• Removing quotes may allow a broader search: (( nonlinear material theory ) or ( nonlinear spectral theory )).
• Adding a wildcard symbol may retrieve word variants: (("nonlinear material theory") or ("nonlinear spectral theory"))*.