Fix LaTeX of free algebras (and repr_lincomb)

[kcrisman]

sage: K.< alpha,b > = FreeAlgebra(SR)
sage: latex(alpha*b)
\alphab

Someone we are not using repr_lincomb correctly, and in fact that function itself looks pretty hackishly defined. I couldn't even find a very good workaround.

See ​the first report at ask.sagemath.

See also: #26606 (same problem for power series in one variable).

[ppurka]

sage/misc/latex.py

@@ -2503 +2503 @@
         '\\alpha'
     """
     if a in common_varnames:
-        return "\\" + a
+        return "\\" + a + "{}"
     elif len(a) == 1:
         return a
     elif is_fname is True:

This fixes the latex, but the raw latex doesn't look good any more.

sage: K.< alpha,b > = FreeAlgebra(SR)      
sage: latex(alpha*b)                       
\alpha{}b

[ppurka]

In fact, what will break if we simply add a space after every such comman variable name?

sage/misc/latex.py

@@ -2503 +2503 @@
         '\\alpha'
     """
     if a in common_varnames:
-        return "\\" + a
+        return "\\" + a + " "
     elif len(a) == 1:
         return a
     elif is_fname is True:

Visually, and in latex, this is probably the simplest fix.

Edit: Ok. Visually, this breaks subscripts and superscripts although the latex is fine.

[mkoeppe]

Still broken in 9.2.beta1.

For repr_lincomb see also #29892.

[gh-DaveWitteMorris]

Related ticket: #26606 (same problem for power series in one variable).

[chapoton]

missing empty line after

+            Check that :trac:`14509` is fixed::

and doctests should be indented, or rather this line should be un-indented

[git]

Branch pushed to git repo; I updated commit sha1. New commits:

​20395b5

Fix doctest

[git]

Branch pushed to git repo; I updated commit sha1. New commits:

​87d3fc7

Fix doctest

[git]

Branch pushed to git repo; I updated commit sha1. New commits:

​ef6c22d

Fix doctest

[git]

Branch pushed to git repo; I updated commit sha1. New commits:

​8d8dd83

Update doctest

[tscrim]

LGTM.