The picture below is a screenshot from the “Using per-item features” video from the “collaborative filtering” section.

I can’t seem to understand what matrix operation has been used here…

It looks like two 2x1 matrices (w^(1) and x^(3)) are multiplied and resulted in a single scalar value (4.95).

Multiplying matrices that are in the same shape is not correct mathematically. Also, the matrices are not square, so this can’t be the result of multiplying their determinant either, since determinant is not defined for non-square matrices.

I’m a bit lost here…

Edit: this operation is also used in the " Collaborative filtering algorithm" video.

Here, w^(1) times x^(1) resulted in 5. I would appreciate it if anyone could name this matrix operation.