greek
I would like to typeset the capital Greek letter for tau. I used \Tau, but I got error for that:
Undefined control sequence.
My preamble is
\documentclass[10pt,a4paper]{article}
\usepackage[latin1]{inputenc}
\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{amssymb}
I was wondering what goes wrong?
Remove the call of the breqn package.
After the operation I get
Note that subfigure has been obsolete for several years. The official replacement is subfig. Alternatively, use subcaption. Note that the class already loads caption, so subcaption should probably be chosen.
I add a version without \left and right, which don't seem necessary (they rarely are in cases like this) and where the superscripts (n) are similar to the one for Gamma. More importantly, the two consecutive equations are together in a gather environment: two equation environments should never appear immediately after one another.
\begin{subequations}
\begin{gather}
[ \tilde{V}^{-^{(n)}} ] = [ \Gamma^{(n)} ] [ \tilde{V}^{+^{(n)}} ],
\\
[ \tilde{V}^{-\,(n)} ] = [ \Gamma^{(n)} ] [ \tilde{V}^{+\,(n)} ],
\end{gather}
\end{subequations}
In order to accommodate the split formula where you use \left and \right across lines, just remove all of them; for better clarity, some of the fences can be made \big, but not more than that. Note that multline is the environment of choice for this.
\begin{multline}
[V_m^{(n)}(z)]=[ P^{(n)} ]
\bigl( \bigl[ \exp \bigl( -j k ^{(n)} ( z-z_n) \bigr) \bigr] [ \tilde{V}^{+^{(n)}}] + \\
\bigl[ \exp \bigl( +j k ^{(n)} ( z-z_{n+1} ) \bigr) \bigr] [ \tilde{V}^{-^{(n)}} ]\bigr)
\end{multline}
That's the chi in the Symbol Adobe font.
\documentclass{article}
\usepackage{mathptmx}
\usepackage{amsmath}
\newcommand{\upchi}{\text{\usefont{U}{psy}{m}{n}\symbol{'143}}}
\begin{document}
X\upchi
$\upchi+X_{\upchi}$
\end{document}
Best Answer
since many of the greek uppercase letters have the same shape as latin letters, they were not separately provided for tex, for two reasons:
to save space (which was a real consideration in 1980);
because it wouldn't be possible to tell the difference between the two shapes.
the first reason is no longer relevant, of course, but the second is.
go ahead and define your own:
for latex, or
if you're using plain tex.