Perhaps you should try some spelling variations:
"relational systems" » "national systems", "educational systems", "international systems"
"functional systems" » "function systems", "national systems", "educational systems"
"operation system" » "operating system", "operating systems", "generation system"

Try widening your search to All Fields.

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: (((("relational systems") OR ("functional systems"))) OR ("operation system")).
• The words AND, OR and NOT may confuse the search; try adding quotes: "((((\"relational systems\") OR (\"functional systems\"))) or (\"operation system\"))".
• Removing quotes may allow a broader search: (((( relational systems ) OR ( functional systems ))) or ( operation system )).
• Adding a wildcard symbol may retrieve word variants: (((("relational systems") OR ("functional systems"))) or ("operation system"))*.