Perhaps you should try some spelling variations:
"observations theory" » "observations towards"
"operation theory" » "operations theory", "iteration theory", "generation theory"
"observations thereon" » "observations thereof", "observations therein"

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 ((("operation theory") OR ((("observations theory") OR ("observations thereon")))))).
• The words AND, OR and NOT may confuse the search; try adding quotes: "((((\"operator theory\") OR (\"operators theory\"))) or (((\"operation theory\") or (((\"observations theory\") or (\"observations thereon\"))))))".
• Removing quotes may allow a broader search: (((( operator theory ) OR ( operators theory ))) or ((( operation theory ) or ((( observations theory ) or ( observations thereon )))))).
• Adding a wildcard symbol may retrieve word variants: (((("operator theory") OR ("operators theory"))) or ((("operation theory") or ((("observations theory") or ("observations thereon"))))))*.