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