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