Perhaps you should try some spelling variations:
"serie letras de clase." » "serie las de clase.", "serie detras de clase.", "serie letras de clacso.", "serie letras la clase."
"serie actas de clase." » "serie fichas de clase.", "serie cartas de clase.", "serie actas de clacso.", "serie actas la clase."
"serie actas del bases." » "serie fichas del bases.", "serie actas de bases.", "serie actas en bases.", "serie actas del babel.", "serie cartas del bases."

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

• Performing a fuzzy search may retrieve terms with similar spellings: (((("serie actas de clase.") OR ("serie letras de clase."))) or ("serie actas del bases."))~.
• If you are trying to use Boolean operators, they must be ALL CAPS: (((("serie actas de clase.") OR ("serie letras de clase."))) OR ("serie actas del bases."))*.
• The words AND, OR and NOT may confuse the search; try adding quotes: "((((\"serie actas de clase.\") OR (\"serie letras de clase.\"))) or (\"serie actas del bases.\"))*".
• Removing quotes may allow a broader search: (((( serie actas de clase. ) OR ( serie letras de clase. ))) or ( serie actas del bases. ))*.