简单条件方程

Question

我测试了一些解决方案，但没有得到简单的结果。美学上我想使用箭头。而不是括号。公式后面是带有条件解释的文本。你对此有什么简单的建议吗？谢谢

\documentclass[a4paper]{article} 

\usepackage[T1]{fontenc} 
\usepackage[italian]{babel}
\usepackage{amsmath}
\usepackage{tikz}
\usepackage{mwe}
\usepackage{lmodern}
\makeatletter
\DeclareMathSizes{\@xpt}{\@xpt}{5}{4}
\DeclareMathSizes{\@xipt}{\@xipt}{6}{5}
\DeclareMathSizes{\@xiipt}{\@xiipt}{6}{5}
\DeclareMathSizes{\@xivpt}{\@xivpt}{\@xpt}{6}
\makeatother

\begin{document}
$
\alpha = m\frac{l'\,z}{l_{0}}\;
\begin{cases}
>0\\=0\\<0
\end{cases}
$

$
\alpha = m\frac{l'\,z}{l_{0}}
\begin{array}{c}
\nearrow >0\\
\rightarrow = 0\\
\searrow < 0\\
\end{array}
$
\end{document}

------------更新------------------

这就是我想要得到的：

\begin{center}
    \begin{subequations}
        \begin{minipage}{.32\textwidth}
            \centering
            \begin{equation}
            \begin{aligned}
            P_{G} = \frac{1}{\rho_{z}}gz_{,i}(\overline{\rho'u_{i}'})\label{eq097.4a}
            \end{aligned}
            \end{equation}
        \end{minipage}\hfil
        \begin{minipage}{.30 \textwidth}
            %\centering
            \begin{equation}
            \begin{aligned}
            P_{G} = \frac{1}{\rho_{z}}g(\overline{\rho' w'})\label{eq097.4b}
            \end{aligned}
            \end{equation}
        \end{minipage}\hfill
        \begin{minipage}{.35 \textwidth}
            \begin{equation}
            \begin{aligned}
            P_{G} = g\frac{\overline{\theta' w'}}{\theta_{0}}\label{eq097.4c}\;
            \begin{array}{c@{\;}l}
            &>0\\[-.75ex]
            \nearrow\\ 
            \rightarrow &= 0\\
            \searrow\\[-.5ex]
            &< 0
            \end{array}
            \end{aligned}
            \end{equation}
        \end{minipage}%\hfill
    \end{subequations}
\end{center}

Answer 1

在环境中使用两列array：

\documentclass{article}

\begin{document}
$
\alpha = m\frac{l'\,z}{l_{0}}
\begin{array}{c@{\;}l}
  &>0\\[-.75ex]
  \nearrow\\ 
  \rightarrow &= 0\\
  \searrow\\[-.5ex]
  &< 0
\end{array}
$
\end{document}

Answer 2

例如，对于第二个公式，没有左括号，我的建议是使用包tikz-cd。

\documentclass{article}
\usepackage{tikz-cd}
\usepackage{amsmath}
\begin{document}
\begin{tikzcd}[row sep=.4cm, column sep=.5cm]
& >0 \\
{\alpha = m\dfrac{l'\,z}{l_{0}}}
\arrow[ru] \arrow[r] \arrow[rd] & =0 \\
& <0
\end{tikzcd}
\end{document}

Answer 3

这里使用两个单独的堆栈完成，以便箭头垂直间隙可以与条件堆栈的基线跳跃不同。

\documentclass[a4paper]{article} 
\usepackage[T1]{fontenc} 
\usepackage[italian]{babel}
\usepackage{amsmath}
\usepackage{tikz}
\usepackage{mwe}
\usepackage{lmodern}
\makeatletter
\DeclareMathSizes{\@xpt}{\@xpt}{5}{4}
\DeclareMathSizes{\@xipt}{\@xipt}{6}{5}
\DeclareMathSizes{\@xiipt}{\@xiipt}{6}{5}
\DeclareMathSizes{\@xivpt}{\@xivpt}{\@xpt}{6}
\makeatother
\usepackage[usestackEOL]{stackengine}
\stackMath
\newcommand\arrowstack{\,\ensurestackMath{\stackengine{\Sstackgap}{%
    \stackengine{\Sstackgap}{\rightarrow}{\nearrow}{O}{c}{F}{F}{S}%
  }{\searrow}{U}{c}{F}{F}{S}%
}}
\begin{document}
\[
\setstackgap{L}{16pt}% GOVERNS THE CONDITIONS STACK BASELINESKIP
\setstackgap{S}{2pt}% GOVERNS THE ARROW STACK GAP
\alpha = m\frac{l'\,z}{l_{0}}
\arrowstack
\Centerstack[l]{{}>0\\{}=0\\{}<0}
\]
\end{document}