Perhaps you should try some spelling variations:
imputation techniques » implantation technique, augmentation techniques, segmentation techniques
reduction techniques » reduction technique
co imputation » co limitation
no2 reduction » h2o2 reduction, _ reduction
based no2 » based n
based co » based code, based two, based _

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

• Performing a fuzzy search may retrieve terms with similar spellings: (( learning-based co imputation techniques ) or ( learning-based no2 reduction techniques ))~.
• If you are trying to use Boolean operators, they must be ALL CAPS: (( learning-based co imputation techniques ) OR ( learning-based no2 reduction techniques )).
• Adding a wildcard symbol may retrieve word variants: (( learning-based co imputation techniques ) or ( learning-based no2 reduction techniques ))*.