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