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