Perhaps you should try some spelling variations:
"functional systems" » "function systems", "national systems", "educational systems"
"operations systems" » "operations system", "operational systems", "operating systems"
"operation systems" » "operation system", "operational systems", "cooperation 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")).
• The words AND, OR and NOT may confuse the search; try adding quotes: "((((\"operation systems\") OR (\"operations systems\"))) or (\"functional systems\"))".
• Removing quotes may allow a broader search: (((( operation systems ) OR ( operations systems ))) or ( functional systems )).
• Adding a wildcard symbol may retrieve word variants: (((("operation systems") OR ("operations systems"))) or ("functional systems"))*.