Perhaps you should try some spelling variations:
"nonlinear material theory" » "nonlinear materials theory", "nonlinear matrix theory", "nonlinear partial theory"
"line spectra theory" » "line spectral theory", "line spectrum theory", "line spectra theorem", "wave spectra 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 ("line spectra theory"))*.
• Removing quotes may allow a broader search: (( nonlinear material theory ) or ( line spectra theory ))*.