Perhaps you should try some spelling variations:
"operational systems" » "operating systems", "conversational systems", "computational systems"
"relational systems" » "national systems", "educational systems", "functional systems"
"operation system" » "operating system", "operating systems", "generation 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: (((("operational systems") OR ("operation system"))) OR ("relational systems"))*.
• Removing quotes may allow a broader search: (((( operational systems ) or ( operation system ))) or ( relational systems ))*.