Perhaps you should try some spelling variations:
"world back technical paper" » "world back technical papers", "world pack technical paper", "world rack technical paper", "world stack technical paper", "world black technical paper", "world jack technical paper", "world lack technical paper"
"worlds back technical paper" » "worlds back technical papers", "worlds pack technical paper", "worlds rack technical paper", "worlds stack technical paper", "worlds black technical paper", "worlds banken technical paper", "worlds based technical paper"
"world bank technical paper" » "world bank technical papers", "world banks technical paper", "world lanka technical paper", "world bands technical paper"

Try widening your search to All Fields.

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• If you are trying to use Boolean operators, they must be ALL CAPS: (((("world back technical paper") OR ("worlds back technical paper"))) OR ("world bank technical paper")).
• The words AND, OR and NOT may confuse the search; try adding quotes: "((((\"world back technical paper\") OR (\"worlds back technical paper\"))) or (\"world bank technical paper\"))".
• Removing quotes may allow a broader search: (((( world back technical paper ) OR ( worlds back technical paper ))) or ( world bank technical paper )).
• Adding a wildcard symbol may retrieve word variants: (((("world back technical paper") OR ("worlds back technical paper"))) or ("world bank technical paper"))*.