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