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