"focused on" is fine in the right grammatical context, but it doesn't fit the structure of the test sentence. " has to use the -ing form of the verb. It's not specifically an issue with the verb "focus".
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Stenka25But how come 145,000 sentences detected with "rather than focused" on Google?Again, there is no problem with "rather than focused" in the right context. A few random examples from Google:
Stenka25But I still cannot but think "focused" can be all rightIn your original sentence, "focused" is definitely wrong.
Stenka25and also in the Webster's Learner's Dictionary, there isn't any saying that we should use only -ing form after 'rahter than.'I did not say that you always have to use the "-ing" form of a v