Perhaps you should try some spelling variations:
"interaction theory" » "intersection theory", "integration theory", "introduction theory"
"operators theory" » "operator theory"
"operation theory" » "operations theory", "operator theory", "generation theory"
"iteration theory" » "integration theory", "vibration theory", "interpolation 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: (("operators theory") OR ((("operation theory") OR ((("iteration theory") OR ("interaction theory")))))).
• The words AND, OR and NOT may confuse the search; try adding quotes: "((\"operators theory\") or (((\"operation theory\") or (((\"iteration theory\") OR (\"interaction theory\"))))))".
• Removing quotes may allow a broader search: (( operators theory ) or ((( operation theory ) or ((( iteration theory ) OR ( interaction theory )))))).
• Adding a wildcard symbol may retrieve word variants: (("operators theory") or ((("operation theory") or ((("iteration theory") OR ("interaction theory"))))))*.