Perhaps you should try some spelling variations:
"observations theory" » "observations thereon", "observations towards"
"operation theory" » "operations theory", "iteration theory", "generation theory"
"options theory" » "optics theory", "equations theory", "functions 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 ((((("observations theory") OR ("options theory"))) OR ("relations theory")))))).
• Removing quotes may allow a broader search: (( operator theory ) or ((( operation theory ) or ((((( observations theory ) or ( options theory ))) or ( relations theory )))))).
• Adding a wildcard symbol may retrieve word variants: (("operator theory") or ((("operation theory") or ((((("observations theory") or ("options theory"))) or ("relations theory"))))))*.