ribet@maalox.berkeley.edu (Kenneth A. Ribet) (10/10/89)
Hello, everyone. I'm posting this question to find out whether
the peculiarity of TeX which I ran into this morning is a well known
phenomenon.
Consider the expressions ${A_B}_C$ and ${\tilde A}_B$. They pose
no problems for TeX. However, TeX refuses to process the expression
${{\tilde A}_B}_C$,
claiming that there is a ``double subscript.'' The only workaround
for this that I know is to embed the $\tilde A$ in a \hbox.
Presumably, this behavior is explained by the rules in Appendix G
of the TeXbook; even so, I find it a bit illogical.
Am I doing something wrong? Is my difficulty familiar to everybody?
-ken ribet, UC Berkeley Math Department
...ucbvax!math!ribet ribet@math.berkeley.eduribet@maalox.berkeley.edu (Kenneth A. Ribet) (10/11/89)
In article <1989Oct9.214708.8873@agate.berkeley.edu> I wrote: >Consider the expressions ${A_B}_C$ and ${\tilde A}_B$. They pose >no problems for TeX. However, TeX refuses to process the expression > ${{\tilde A}_B}_C$, >claiming that there is a ``double subscript.'' The only workaround >for this that I know is to embed the $\tilde A$ in a \hbox. > >Presumably, this behavior is explained by the rules in Appendix G >of the TeXbook; even so, I find it a bit illogical. The solution to my difficulty was provided by Joe Weening (weening@Gang-of-Four.Stanford.EDU): >On p. 291 of the TeXbook, when describing the semantics of braces in >math mode, it says "If the resulting math list is a single Acc atom, >however (i.e., an accented quantity), that atom is appended." This is >probably done to make things like {\tilde A}_B come out right; he >wants to make B a subscript of the Acc atom, not of an Ord atom that >would otherwise result from the braces. But it leads to the anomaly >in your case, that > A_B_C fails because of a double subscript > {A_B}_C doesn't fail because {A_B} becomes an atom > {{\tilde A}_B}_C fails because {\tilde A} is an Acc atom, > and {{\tilde A}_B} is also an Acc atom, so > the braces have no net effect >But you can fix it by putting in a {} in various places: > {{}{\tilde A}_B}_C > {{{}\tilde A}_B}_C > {{\tilde A{}}_B}_C > {{\tilde A}{}_B}_C > {{\tilde A}_B{}}_C > {{\tilde A}_B}{}_C I don't like the last alternative, because it makes the subscript $C$ at the same level on the page as the subscript $B$, but the first five solutions seem fine. Thanks, Joe. -ken ribet, UC Berkeley Math Department ...ucbvax!math!ribet ribet@math.berkeley.edu