math-modesymbols
I've just rendered the following:
\documentclass[varwidth=true, border=2pt]{standalone}
\begin{document}$\textbackslash$
\end{document}
which seems to give the same as $n$. Do you know why?
As \textbackslash is for textmode and not for mathmode, I've expected TeX to fail in this situation. Instead, I get
LaTeX Warning: Command \textbackslash invalid in math mode
So why does \textasciitilde in math mode make LaTeX crash but \textbackslash not?
If you are trying to write verbatim short text, you can use \verb+\hi+; for longer verbatim texts you can use the verbatim environment; for further customizable possibilities you can use, for example, the fancyvrb package.
Here is an alternative to your current situation - using an optional argument to specify the derivative order. This way you don't have to define a macro for "each" derivative:
\deriv[<order>]{<func>}{<var>}
Here's a minimal example:
\documentclass{article}
\newcommand{\deriv}[3][]{% \deriv[<order>]{<func>}{<var>}
\ensuremath{\frac{\partial^{#1} {#2}}{\partial {#3}^{#1}}}}
\begin{document}
In text mode there is~\deriv{y}{x} and~\deriv[2]{y}{x}. In display mode there is
\[
\deriv{y}{x}\ \textrm{and}\ \deriv[2]{y}{x}\rlap{.}
\]
\end{document}
The default <order> is empty, implying the first order partial derivative. If you want the default to be 2, modify the definition to read
\newcommand{\deriv}[3][2]{...}
Technically it is possible to use a macro with numbers in them, but the usage is much less intuitive than adding something like an optional argument (as given above). Here's an implementation that now allows you to use \nameuse{deriv2}{y}{x}:
\expandafter\def\csname deriv2\endcsname#1#2{%
\ensuremath{\frac{\partial^2 {#1}}{\partial {#2}^2}}}
\makeatletter
\newcommand{\nameuse}[1]{\@nameuse{#1}}%
\makeatother
The optional argument beats this hands down.
Best Answer
The definition of
\textbackslashwhen the OT1 encoding is used isBoth the first and the third token can't be typed by a user without some devious trick. However, the definition of
\OMS-cmdis equivalent toThe macro
\@inmathwarnis responsible for the warningAfter that the macro
\OMS\textbackslashis expanded. If we were in text mode, the command would do a font change in a group, selecting a font in the OMS encoding (math symbols), doingand a backslash would appear, because that's what an OMS encoded font has at slot
"6E(hexadecimal).If we're in math mode, the same
\char"6Einstruction would be performed, but here the font change has no effect. So we get the character at position"6Ein the font in math family 0, where annis found. When\char"6Eis found in math mode, TeX does as if it were\mathchar"006Eand in family 0, slot"6Ethere's ann(it's the upright text font and the ASCII code ofnis exactly"6E).Things are different in the T1 encoding, because in this case the definition of
\textbackslashisand the latter macro eventually expands to
\char"5C. In a text font, T1 encoding, at slot"5Cthere's actually a backslash.Here's the explanation. Now, some advice:
Never underestimate warnings and
never use
\text...symbol commands in math mode (don't mistake them with\textrm,\textitor similar, which are obviously legal in math mode).