Perhaps you should try some spelling variations:
del investigation » gel investigation, model investigation, delay investigation
de investigacion » la investigacion
seis estudios » pacis estudios, los estudios, tres estudios
estudios del » estudio del, estudio de, estudos de
estudios de » estudio de

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 de investigacion ) OR ( seis estudios del investigation )).
• Adding a wildcard symbol may retrieve word variants: (( de estudios de investigacion ) or ( seis estudios del investigation ))*.