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