Perhaps you should try some spelling variations:
"functional systems" » "national systems", "educational systems"
"operations systems" » "operations system", "operational systems", "operating systems"
"operation systems" » "operation system", "operational systems", "cooperation systems"
"function systems" » "reaction systems", "detection systems"

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