Perhaps you should try some spelling variations:
"operational systems" » "operating systems", "relational systems", "conversational systems"
"functional systems" » "function systems", "educational systems"
"operation system" » "operating system", "operating systems", "generation system"
"national systems" » "rational system", "natural systems", "educational 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 system") OR ("operational systems"))) OR ((("functional systems") OR ("national systems")))).
• Removing quotes may allow a broader search: (((( operation system ) or ( operational systems ))) or ((( functional systems ) or ( national systems )))).
• Adding a wildcard symbol may retrieve word variants: (((("operation system") or ("operational systems"))) or ((("functional systems") or ("national systems"))))*.