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