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