Perhaps you should try some spelling variations:
"operation systems" » "operations systems", "operation system", "operational systems"
"national systems" » "natural systems", "educational systems"
"rational system" » "educational system", "international system"

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: (("operation systems") OR ((("national systems") OR ("rational system"))))*.
• The words AND, OR and NOT may confuse the search; try adding quotes: "((\"operation systems\") or (((\"national systems\") OR (\"rational system\"))))*".
• Removing quotes may allow a broader search: (( operation systems ) or ((( national systems ) OR ( rational system ))))*.