Recomender system
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1. Explain what does it mean "Principal Component Analysis".

2. What are the challenges for building recommender system?

3. Recomender system can be based on: popularity, classification,
   co-occurence. Can you explain what does it mean.

4. Can you explain what does it mean "matrix factorisation" and
   "collaborative filtering"? How it can be used in recomended system.

5. How we can measure effectiveness of the recomender system.