Perhaps you should try some spelling variations:
"operations theory" » "observations theory", "operator theory", "options theory"
"operators theory" » "operator theory"
"operation theory" » "operator theory", "iteration theory", "generation 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 ((("operations theory") OR ("relations theory")))))).
• Removing quotes may allow a broader search: (( operators theory ) or ((( operation theory ) or ((( operations theory ) or ( relations theory )))))).
• Adding a wildcard symbol may retrieve word variants: (("operators theory") or ((("operation theory") or ((("operations theory") or ("relations theory"))))))*.