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