Perhaps you should try some spelling variations:
"seis estudios americanos." » "seis estudios latinoamericanos.", "los estudios americanos.", "dos estudios americanos.", "dios estudios americanos."
"series estudios urbano." » "serie estudios urbano.", "leyes estudios urbano.", "sociales estudios urbano.", "series estudios urbanos.", "series estudio urbano.", "series estudios peruanos."
"tres estudios urbano." » "tres estudios urbanos.", "tres estudio urbano.", "tres estudios peruanos.", "otros estudios urbano.", "los estudios urbano.", "dos estudios urbano."

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

• Performing a fuzzy search may retrieve terms with similar spellings: (((("series estudios urbano.") OR ("tres estudios urbano."))) or ("seis estudios americanos."))~.
• If you are trying to use Boolean operators, they must be ALL CAPS: (((("series estudios urbano.") OR ("tres estudios urbano."))) OR ("seis estudios americanos."))*.
• The words AND, OR and NOT may confuse the search; try adding quotes: "((((\"series estudios urbano.\") OR (\"tres estudios urbano.\"))) or (\"seis estudios americanos.\"))*".
• Removing quotes may allow a broader search: (((( series estudios urbano. ) OR ( tres estudios urbano. ))) or ( seis estudios americanos. ))*.