I’m going to post this because I spent more time than I want to admit on something that could have been 5 minutes and hopefully I can help unstuck someone who like me, didn’t read the regularization portion of the formula correctly.
Pay special attention to the lower and upper limit of the Summations in the formula.
For some reason I thought the regularization was the summations of X and W where r(i,j)=1, that is not the case. So read them carefully and I’m certain that if you made it this far, you’ll do great!