min RMS and quadratic form expression

Hello,
In the lecture 7 (slide 15), I'm stuck with understanding what's the meaning of matrix A and vector c in the sense of that problem.
Capture d’écran 2021-04-20 à 16.14.18.jpg

I don't really see the link between those dimensions (key notion in next slides) and this minimisation problem. I must be missing linear algebra principles, could you help me see better ?

Thanks a lot

Top comment

I would refer you to HW4 Ex2, where you need to find a way to write the RMS expression in matrix form as \( Ab-c \), so as to apply the general formula for least squares. One useful hint is that \( A \) will have one row for each pair \( (u,i) \) where there is a rating in the training set.
I hope this helps!
Best,
MG

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