alignsubscripts
I saw some questions about "double subscript" but I have a weird situation. I have the following parts in my code:
and…
I have used almost the same subscripts in both of them, but the latter one gives a "double subscript" error while the first one works fine. What am I missing there? What is the reason for that error?
Here is my error bar:
The extent to which a "command defined by \newcommand is 'automatically' a single "object" on its own" varies by context.
Certainly such a command is a token, which is an atomic object in TeX's parser. When TeX is reading input, it receives any token as a whole, regardless of its expansion (if any).
It is also an expandable token, which means that it is not an atomic object in TeX's interpreter. In various situations, the input is subject to expansion, in which all expandable tokens are (successively left-to-right) replaced by their meanings.
It is not, as David explains, a "brace group", which is often treated as a single object, particularly when taken as an "argument" to a macro or function-like primitive operator (such as the subscript symbol).
One of the situations in which tokens are expanded is when a subscript symbol is looking for its operand; it expects either a brace group or a single character (or control sequence equivalent to a character). Thus, most \newcommand macros can't be used openly as subscripts, if they expand to more than one token that aren't braced.
I find the notation
${v_{k}}_d$
unclear; readers won't be able to distinguish it from
$v_{kd}$
However, if you really insist on doing it that way, you just have to add a pair of braces; I add what I'd do, instead.
\documentclass{article}
\begin{document}
${v_{k}}_{d}$
${{}\dot{v}_{k}}_{d}$
Better:
$(v_{k})_{d}$
$(\dot{v}_{k})_{d}$
\end{document}
It's a problem with math accent that sometimes shows up; in an expression such as ${\dot{v}_k}$ the outer braces are stripped off by rule, because they contain a single Acc atom; therefore the intuitive
${\dot{v}_k}_d$
gives a Double subscript error. Adding an empty subformula avoids the problem.
Best Answer
The error is caused by the improper double subscript
h_{j)}_0(of typea_b_c). Add brackets to help latex discriminate betweena_{b_c}or{a_b}_c.Note that is similar to the ambiguity in maths: is 2^3^4 supposed to mean 8^4 or 2^81?