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