What's the best way to write subsequences? $x_n_i$ gives an error, while $x_{n_{i}}$ works, but it just looks like $x_{ni}$ (the i isn't lowered enough below the n)…
[Tex/LaTex] Double Subscript for Subsequences
subscripts
Related Solutions
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.
You have to be disciplined and write, if a subscript is needed,
\Zs^{<superscript>}_{<subscript>}
Here's the code:
\documentclass{article}
\makeatletter
\newcommand{\passerby@genericstarred}[1]{%
#1\@ifnextchar^\passerby@eathat{^*}%
}
\def\passerby@eathat#1#2{% #1 is ^
^{*#2}%
}
\newcommand{\definegenericstarred}[1]{%
\@for\next:=#1\do{%
\expandafter\edef\csname\next s\endcsname{%
\noexpand\passerby@genericstarred{\next}}%
}%
}
\makeatother
\definegenericstarred{X,Y,Z,W}
\begin{document}
$\Xs+\Zs^T+\Ws+\Ys^2$
\end{document}
The same with LaTeX3:
\documentclass{article}
\usepackage{xparse}
\ExplSyntaxOn
\NewDocumentCommand{\definegenericstarred}{m}
{
\clist_map_inline:nn { #1 }
{
\cs_new_protected:cpn { ##1s } { \passerby_generic_starred:n { ##1 } }
}
}
\cs_new_protected:Npn \passerby_generic_starred:n #1
{
#1 % print the main letter
\peek_catcode_remove:NTF ^ % check if a ^ follows
{ \passerby_add_star:n } % yes, add the * to the explicit superscript
{ ^{*} } % no, just add the *
}
\cs_new_protected:Npn \passerby_add_star:n #1
{
^{*#1} % the final superscript
}
\ExplSyntaxOff
\definegenericstarred{X,Y,Z,W}
\begin{document}
$\Xs+\Zs^T+\Ws+\Ys^2$
\end{document}
Best Answer
I'm a bit puzzled by your statement that
$x_{n_i}$would create output which "just looks like" that of$x_{ni}$-- this is not the case in the following MWE (minimum working example). I have a hunch that what you want is$x_{n_i}$, but read on.Clearly, the first expression,
$x_{ni}$, can't be what you want. Note that${x_n}_i$and$x_{n_i}$are both valid expressions from a purely syntactic point of view. However, they do not create the same output. In the former case, the characteriis both lowered by a smaller amount and has a larger font size than is the case in the latter. (To be a bit TeXnical, in the first two expressions above,nandiare both typeset in "scriptstyle", whereas in the third expressionnis in scriptstyle andiis in "scriptscriptstyle". For Computer Modern math fonts, "scriptstyle" is 30% linearly reduced from "textstyle", and "scriptscriptstyle" is 30% reduced from "scriptstyle" -- or ca 50% linearly reduced from "textstyle". Thus, if the textstyle font size is 10pt, scriptsize is 7pt and scriptscriptsize is 5pt.)These differences in appearance are, of course, not accidental: in the middle expression above, the symbol
iis a subscript/index to the subformula$x_n$, whereas in the final expressioniindexesnwhich, in turn, indexesx. Put differently, in the final expression${n_i}$is a subformula that serves to indexx.If there's any chance for ambiguity as to which characters are supposed to index which other characters, you should not hesitate to use parentheses, brackets, or braces -- or whatever grouping symbols are appropriate in your math writing style -- to clarify the intended meaning of your writing.
Addendum, prompted by a follow-up communication from the OP. As the image above illustrates, there's not much visual difference in the appearance of the first and second cases, i.e., of
$x_{ni}$and${x_n}_i$. In both cases, (a) thenandicharacters are inscriptsize, and (b) thei's are set below the baseline by the same amount. The only visual difference, then, is that TeX sets thenin the second case in so-called "cramped subscript" mode, which differs from the "ordinary subscript" position by a (very) small amount.