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