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