I would use articles here. You're using 'correlation' as a count noun; there are two correlations here: temporal and spacial. The noun has its plural form, hence articles.
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leiwang81two major challenges: temporal correlation(1) and spatial correlation. Temporal correlation(2) refers to .. and spatial correlation refers to ...correlation is uncountable. It is correct without an article.
leiwang81A cube has two parts: an input part (3) and an output part. The input part(4) is defined to be ...
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leiwang81So IYes. That's correct.knewknow it is correct. They didn't use "the" there.
leiwang81What is the difference between zero article and "the" at (2)."the correlation" is used if there is a specific correlation which has been mentioned earlier in the text. You