Perhaps you should try some spelling variations:
"aberration theory" » "aberration test"
"generation theory" » "generation theorem", "iteration theory", "generation third"
"generative theory" » "generalized theory", "integrative theory", "alternative theory"
"operation theory" » "operations theory", "iteration theory", "operation theatre"
"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 ("aberration 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 (\"aberration theory\"))) or (((\"generation theory\") or (\"generative theory\"))))))".
• Removing quotes may allow a broader search: (( operator theory ) or ((((( operation theory ) OR ( aberration theory ))) or ((( generation theory ) or ( generative theory )))))).
• Adding a wildcard symbol may retrieve word variants: (("operator theory") or ((((("operation theory") OR ("aberration theory"))) or ((("generation theory") or ("generative theory"))))))*.