By b &#x2F; A he is implying a horizontal fraction bar with b above and A below. Of course, since matrix multiplication is not commutative, it’s probably better to write something like &#x2F;A * b (or Matlab uses the cute notation A \ b) instead of b &#x2F; A, but the intention is fairly clear from context.<p>(In general, most of our mathematical notation conventions are designed for the real number system, and start falling over when we have “number-like objects” which behave a bit differently.)