I know the question is in a techninal context, but my main problem is the mathematical approach behind it. Hopefully I am right here with my question.
I just read a way to implement a recommendation system but I don't really unterstand the mathematical approach behind it (link).
The author uses three vectors to generate a prediction for a item. In his last image example he uses the "SUMPRODUCT"-function from Microsoft Excel which just build the sum of the products from his three vectors. He explain it mathematically as simple dot product, but is it true? I thought a dot product like this is only for two vectors possible. I know, it exist the triple product method, but in this case he would need the cross product of two vectors. Is here any mathematical approach to legitimize/explain this technique?
Thanks for your help and sorry for my bad english!