0
$\begingroup$

I'm following the solution to exercise 4.6 from https://tommyodland.com/files/edu/bishop_solutions.pdf

The equality $$\Sigma_n(w^Tx_n -w^Tm)x_n = \Sigma_n(x_nx_n^T -x_nm^T)w$$ doesn't make sense to me. I understand that one can write $(w^Tx_n)x_n = x_n(w^Tx_n) = x_n(x_n^Tw)$, but the step $x_n(x_n^Tw) = (x_nx_n^T)w$ should be false.

$\endgroup$
  • 8
    $\begingroup$ Why should it be false? Isn't it simply associativity of matrix multiplication? $\endgroup$ – Bungo Sep 16 at 7:37
  • 2
    $\begingroup$ $(w^Tx_n) x_n=x_n(w^Tx_n) $ is more confusing to me, because you don't have commutativity of matrices in general, you always have associativity though. $\endgroup$ – kingW3 Sep 16 at 9:47
  • 1
    $\begingroup$ @kingW3 $(w^T x_n)$ is presumably a scalar, so it commutes with any matrix, and associativity works as long as the dimensions are compatible. Also because it is a (presumably real) scalar, it equals its own transpose, which is why $w^T x_n = x_n^T w$. $\endgroup$ – Bungo Sep 16 at 21:39

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Browse other questions tagged or ask your own question.