Perhaps you should try some spelling variations:
"seis estudio de investigacion" » "seis estudios de investigacion", "el estudio de investigacion", "de estudio de investigacion"
"de estudios la investigacion" » "de estudios de investigacion", "de estudios en investigacion", "de estudios y investigacion"

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: (("de estudios la investigacion") OR ("seis estudio de investigacion")).
• Removing quotes may allow a broader search: (( de estudios la investigacion ) or ( seis estudio de investigacion )).
• Adding a wildcard symbol may retrieve word variants: (("de estudios la investigacion") or ("seis estudio de investigacion"))*.