Perhaps you should try some spelling variations:
"series estudios americanos." » "series estudios latinoamericanos.", "serie estudios americanos.", "seis estudios americanos.", "leyes estudios americanos."
"tres estudios hispanos." » "tres estudios hispano.", "tres estudios hispanicos.", "tres estudios espanoles.", "otros estudios hispanos.", "los estudios hispanos.", "dios estudios hispanos."
"dos estudios urbanos." » "los estudios urbanos."

You may be able to get more results by adjusting your search query.

• Performing a fuzzy search may retrieve terms with similar spellings: (((("dos estudios urbanos.") or ("tres estudios hispanos."))) or ("series estudios americanos."))~.
• If you are trying to use Boolean operators, they must be ALL CAPS: (((("dos estudios urbanos.") OR ("tres estudios hispanos."))) OR ("series estudios americanos."))*.
• Removing quotes may allow a broader search: (((( dos estudios urbanos. ) or ( tres estudios hispanos. ))) or ( series estudios americanos. ))*.