[Jprogramming] Rank of matrix divide

[Raul Miller]

On Sat, May 21, 2022 at 3:39 AM Adrien Mathieu
<adrien.lc.mathieu at gmail.com> wrote:
> It took me some time to understand why you were mentioning A%.A, as
> matrix multiplication is not commutative. Turns out, by reading the
> documentation, that x%.y is actually (%.y) +/ .* x, not x +/ .* %.y as I
> implicitly assumed. Btw, why is that? Why does dividing on the right by
> y actually multiplies on the left by the inverse? Isn't that weird?

Hmm...

Well, one answer is that that's how it's documented --
https://www.jsoftware.com/help/dictionary/d131.htm

But another answer might have to do with how E.E. McDonnell had
originally implemented this -- (fourth paragraph of the introduction
at https://www.jsoftware.com/papers/eem/sqrt1.htm#... E.E. McDonnell
spent a lot of time working on x-ray crystallography techniques, and
also had some formative influences on J. See
https://dl.acm.org/doi/10.1145/206944.206985 for example. And, the
techniques for extracting crystal structure from x rays are...
interesting.)

But, also, I guess an underlying problem is that +/ .* is not commutative.

I hope this helps,

-- 
Raul