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