Perhaps you should try some spelling variations:
"collinear particles theory" » "collider particle theory", "colloidal particles theory", "colliding particles theory", "collinear particle theory", "collinear particles through"
"nonlinear material theory" » "nonlinear materials theory", "nonlinear matrix theory", "nonlinear partial 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 ("collinear particles theory")).
• Removing quotes may allow a broader search: (( nonlinear material theory ) or ( collinear particles theory )).
• Adding a wildcard symbol may retrieve word variants: (("nonlinear material theory") or ("collinear particles theory"))*.