[NTG-context] Re: equation to right margin

10 Oct 2024

      Hi Mikael,

My personal experience with numbered equations gathered with a big left curly brace (assuming that the numbers are put to the right of formulas) is that it is very useful (or indispensable…) to be able to number each equation and be able to refer to them. For instance, when one writes a partial differential equation, usually there are boundary conditions, initial conditions and such, which may take 3, 4 or 5 lines of equations (some authors may even have more than that, but I personally avoid it…). It is useful to be able to refer to each of these equations, either by numebring them as subformulas, or numebering them sequentially. It is true that in this case, that is when each line has a number, usually one does not number the whole system of equations gathered by the big left brace. The optimal solution would be to have the option to number each line, or to number them as subformulas.

For instance in the following minimal example (based on one of your answers some time ago), the reference to « eq:Robin » does not work and the numbers of each line of the PDE problem are not correctly aligned to the right.

% begin example alignedcases.tex

\definemathsimplealign[pdeproblem][
left={\startmathfenced[cases]},
right=\stopmathfenced,
align={1:right,2:left,3:left},
strut=yes,
]

\define[1]\eqref{(\in[eq:#1])}
\starttext
A function $u$ is said to be harmonic in $\Omega$ if
\placeformula[eq:Harmonic]
\startformula
\Delta u = 0 \quad\text{in }\, \Omega.
\stopformula
For a given potential $V$ defined on $\partial\Omega$ and a smooth function $\phi$ defined on $\partial\Omega$, study here the PDE
\placeformula 
\startformula
\startpdeproblem
\NC - \Delta u \NC = 0 \NC \qquad\text{in } \Omega \NR[eq:Harmonic-1]
\NC \frac{\partial u}{\partial {\bi n}} \NC = V u + \phi \NC
\qquad\text{on } \partial\Omega \NR[eq:Robin]
\stoppdeproblem
\stopformula

In the above PDE problem, the boundary condition \eqref{eq:Robin} may be replaced by a nonlinear condition such as
\startformula
\frac{\partial u}{\partial {\bi n}} \NC = V u + u^3 \qquad\text{on } \partial\Omega,
\stopformula
while the notion of a harmonic function \eqref{Harmonic} may be replaced with 
\startformula
-{\rm div}(a(x)\nabla u) = 0 \quad\text{in }\, \Omega.
\stopformula

\stoptext

% end example alignedcases.tex

Best regards: Otared
...
On 10 Oct 2024, at 07:32, Mikael Sundqvist  wrote:
Hi,
A. Bowen: The cases are meant for when you divide something into
cases, as in the example below with the absolute value. It is a kind
of inline part of a formula, and I would claim that when you number an
equation, you do not number one case of a cases, but the equation. If
you refer to it, you talk abou "the second line in (1)" if you need to
specify. I also get a bit curious, are you editor of some math journal
that is typeset by ConTeXt?
A. Braslau: I agree that it is common to put the brace on the left,
but I insist that it makes more sense to put it on the right _if_ you
have equation numbers on the right. If equation numbers are to the
left, it makes sense to put the brace on the left. Nevertheless, to
put the brace on the left side and numbering on the right is possible
(see example below). But the equation number in this case is sitting
in between the two equations, and that is disturbing. If there would
be three lines, one could argue that it would sit on the middle one.
Or not distinguish it from the case where it sits on the middle line.
The good thing is of course that one can do as one please.
\starttext
\startformula
 \abs{x} =
 \startcases
   \NC  x, \NC x > 0; \NR
   \NC  0, \NC x = 0; \NR
   \NC -x, \NC x < 0. \NR
 \stopcases
 \numberhere[eq:foo]
\stopformula
\startformula
 \startalign
   [location=packed,fences=cases]
   \NC \[21-(9 - n^2)\NC =□^\prime\]\NR
   \NC  n^2+12 \NC=□^\prime.        \NR
 \stopalign
 \numberhere[eq:foo]
\stopformula
\stoptext
/Mikael
On Thu, Oct 10, 2024 at 2:21 AM Alan Braslau via ntg-context
 wrote:
...
It is common, when writing a system of equations, to put a brace on the
left. It could make sense to number this system of equations with a
single number, on the right and aligned with the right margin. It would
also make sense for this number to be aligned with the middle of the
two equations of the system.
A brace on the right would not introduce the grouping as a system of
equations, rather as a pair of equations sharing a common number.
Alan
On Wed, 9 Oct 2024 12:20:00 -0400
Alan Bowen  wrote:
...
I understand. The reasoning makes sense to me and, as editor of a
paper in which this appears, I will ask the author to consider
dropping the brace.
I wonder of there is some similar thinking underlying the fact that
\startcases... puts the equation number so close to the equation
itself and not at a margin (typically right).
Again, many thanks, Mikael.
Alan
On Wed, Oct 9, 2024 at 12:12 PM Mikael Sundqvist 
wrote:
...
Hi,
I think what you ask for was never supported in ConTeXt, for a
reason. The idea is that either one groups a set of equations with
a brace and numbers them with one number, or one does not group,
and then one can number each line separately/independently.
Grouping on the left with a brace and numbering one line to the
right is, even if we see that often, not really logical.
If there is a large request for doing this, I can talk to Hans about
it, maybe it is easy on the code side to support it. But I remember
from when we were messing with numbering, and in particular when
there are alignments, that it is a bit complicated.
/Mikael
On Wed, Oct 9, 2024 at 5:19 PM Alan Bowen 
wrote:
...
Thank you, Mikael. That is very helpful and much appreciated.
What I now have is:
\startplaceformula
 \startformula
   \startalign
     \NC \[21-(9 - n^2)\NC =□^\prime\]  \NR[eq:1]
     \NC  n^2+12 \NC=□^\prime.          \NR[eq:2]
   \stopalign
 \stopformula
\stopplaceformula
which put the numbers at the right margin,as I wanted.
So how do I get a brace on the left rather than on the right?  Or
is
there documentation that explains this?
...
Alan
On Wed, Oct 9, 2024 at 10:22 AM Mikael Sundqvist

wrote:
...
...
Hi,
On Mon, Oct 7, 2024 at 11:45 PM Alan Bowen

wrote:
...
> 
> I have the following which works, except for the fact that the
equation number does not go to the right margin.
> 
> \startplaceformula
> \startformula[align=middle, prefix=no]
> \startcases[distance=2pt]
> \NC \[21-(9 - n^2)\NC =□^\prime\]\NR
> \NC\hfill n^2+12 \NC=□^\prime.\NR[eq04]
> \stopcases
> \stopformula
> \stopplaceformula
> 
> I expect that I have missed something pretty basic here, so any
guidance will be greatly appreciated.
Maybe I misunderstand you, but I do not think cases is right
here. Also, are you sure you want to group two equations but
only number one of them.
If I understand you correctly, I'd try with one of these,
depending on if citing the group or one of the equations.
\starttext
\startformula
 \startalign[location=packed,fences=sesac]
   \NC \[21-(9 - n^2)\NC =□^\prime\]\NR
   \NC  n^2+12 \NC=□^\prime.        \NR
 \stopalign
 \numberhere[eq:foo]
\stopformula
\startplaceformula
 \startformula
   \startalign
     \NC \[21-(9 - n^2)\NC =□^\prime\]  \NR%[eq:bar-a]
     \NC  n^2+12 \NC=□^\prime.          \NR[eq:bar-b]
   \stopalign
 \stopformula
\stopplaceformula
\stoptext
/Mikael

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maillist : ntg-context@ntg.nl / https://mailman.ntg.nl/mailman3/lists/ntg-context.ntg.nl
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___________________________________________________________________________________
Otared Kavian
e-mail: otared@gmail.com
Phone: +33 6 88 26 70 95