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