I just saw this question but it doesn't answer mine. How do I define a function something like
f(x) = { bla bla if x<0
bla bla if x>0
The above question doesn't answer how to write f(x) before the bracket
Please help.
[Tex/LaTex] How to define a piecewise function in latex
equationsmath-mode
Related Solutions
This situation is exactly what the cases environment from the amsmath package was designed for. If you put
\usepackage{amsmath}
in your preamble then you can wite
\[ \begin{cases}
0 & x\leq 0 \\
\frac{100-x}{100} & 0\leq x\leq 100 \\
0 & 100\leq x
\end{cases}
\]
to adapt your example with the correct size bracket.
As for not being able to use TeX-like syntax to write equations on this site: this is a deliberate choice. Most of the time we actually want to talk about the TeX source here rather than the equations (or whatever) it produces; and there are times when it could be actively confusing to have a TeX-like process to produce images that isn't actually TeX (in its full, Turing-complete, confusing glory...)
(See the discussion on meta for more on this, if you're interested.)
One possibility: I used \DeclarePairedDelimiterX from the mathtools package to define a \MeijerM command with three arguments which is responsible to typeset the delimited matrix; then I defined \MeijerG having eight arguments (the first one is optional and will be passed as the optional argument to \MeijerM); using the \WithSuffix command from the suffix package to provide the starred version \MeijerG*:
\documentclass[11pt]{article}
\usepackage{suffix}
\usepackage{mathtools}
\DeclarePairedDelimiterX\MeijerM[3]{\lparen}{\rparen}%
{\begin{smallmatrix}#1 \\ #2\end{smallmatrix}\delimsize\vert\,#3}
\newcommand\MeijerG[8][]{%
G^{\,#2,#3}_{#4,#5}\MeijerM[#1]{#6}{#7}{#8}}
\WithSuffix\newcommand\MeijerG*[7]{%
G^{\,#1,#2}_{#3,#4}\MeijerM*{#5}{#6}{#7}}
\begin{document}
\[
\MeijerG*{m}{n}{p}{q}{a_1, \dots, a_p}{b_1, \dots, b_q}{z}\quad
\MeijerG[\big]{m}{n}{p}{q}{a_1, \dots, a_p}{b_1, \dots, b_q}{z}\quad
\MeijerG[\Bigg]{m}{n}{p}{q}{a_1, \dots, a_p}{b_1, \dots, b_q}{z}
\]
\end{document}
The size of delimiters in the second and third examples is obviously wrong, but I just included them to test the functionality of the defined commands. Also, I used simple sub/superscripts to typeset the first four arguments, but of course you can use one of your proposed variants instead.
Best Answer
LaTeX is very flexible. Although it is not WYSIWYG you can write it in a very logical way. So if you want to have to have f(x) precede your cases, just write it that way. Here with the example of the linked question.