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