this is the title of this post
do the pair of vertical lines in empirical entropy formula mean L1 norm?
in this case, "do the pair of vertical lines" or "does the pair of vertical lines" which one is grammatical?
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"Pair" is singular, hence "Does the pair ..." is correct.
It's worth noting that number disagreement is a common native speaker error, and that we can often slide from speaking of a singular grouping to the plural objects and back without noticing.