# Why is this NP complete?

I am looking at the diverse subset problem in Kleinberg and Tardos, shown in the image:

Why can't we give a polynomial time algorithm for this? Cant we iterate through each person a, and then each item i, and then see for each person b $$\neq$$ a if b has bought item i, then conclude that a and b cannot be diverse? And if we get through all items and no item that a has bought has been bought by b therefore implies they're diverse? Then we do this with each customer. What's wrong with this approach?

• Don't use images as main content of your post. This makes your question impossible to search and inaccessible to the visually impaired; we don't like that. Please transcribe text and mathematics (note that you can use LaTeX) and don't forget to give proper attribution to your sources! Commented Nov 28, 2018 at 22:47
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• Hint, $k$ could be something like $m/4$. Your brute force searching may run exponential steps. Commented Nov 29, 2018 at 4:45

The algorithm you described isn't polynomial time because it has to consider every possible subset of customers (they are $$2^m$$ of these).
HINT: To do the reduction, consider how you would model $$A$$ as a graph and vice-versa. Once you've done that, consider how a "diverse set" is related to a vertex cover in the graphs.