See the page Nr. 2 in the document ortomatem.pdf refered in the Lyx's Math
There is no such reference.
I googled this:
There the also using round brackets on page 2 (Disposición de fórmulas aisladas)
page and 13
They write there for example
This is inexact wrong because this could mean
cos(2)*v*t or cos(2*v)*t
Moreover this is against the the character omitting rules, see below.
in the same document;and the reference to the ISO 31 in the page Nr. 18 about
the round brackets.
In the norm it is defined that you _can_ but needn't to omit the brackets. But omitting the brackets
leads to inexact result:
In the first case I meant tan(2)*p*t, in the second I meant tan(2*p)*t. So you
see the problem.
Besides this, look at
all other Wikipedia entries state the same. Typesetting rules are important to be able to
communicate with each other. You can take any math book, look at webpages, school books, and also
another LyX user told you as mathematician that your notation is incorrect. So it's up to you to
change it or to ignore this.