Perhaps you should try some spelling variations:
observations towards » observations theory
observations taken » observations thereon, observations tending
operation theory » operations theory, iteration theory, generation theory
operator » operators

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 ( ((observations towards) OR (observations taken)) )))).
• The words AND, OR and NOT may confuse the search; try adding quotes: "(( operator theory ) or ((( operation theory ) or ( ((observations towards) OR (observations taken)) ))))".
• Adding a wildcard symbol may retrieve word variants: (( operator theory ) or ((( operation theory ) or ( ((observations towards) OR (observations taken)) ))))*.