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