在单一环境中以不同方式对齐多条方程线

Question

我不确定如何在标题中解释它，但这是我想做的：

我希望能够对齐正确的箭头，而不必像我在代码中所做的那样将它们分成两个不同的环境：

\documentclass[fleqn, 12pt]{article}
\usepackage[utf8]{inputenc}
\usepackage{amsmath, amsthm, amssymb, mathtools}
\usepackage[margin=1.25in]{geometry}
\setlength{\mathindent}{0pt}
\setlength{\jot}{\baselineskip}
\usepackage[nodisplayskipstretch]{setspace} 
\setstretch{1.25} 

\begin{document}
\begin{align*}
    \Rightarrow k \textrm{ is even } (k=2n)
        \Rightarrow z_k=i^{2n}=(-1)^n 
            &\Rightarrow n \textrm{ is even}
                \Rightarrow z_k=1+0i \\
            &\Rightarrow n \textrm{ is odd}
                \Rightarrow z_k=-1+0i \\
\end{align*}
\vspace{-2cm}
\begin{align*}
    \Rightarrow k \textrm{ is odd } (k=2n+1)
        \Rightarrow z_k=i^{2n+1}=i^{2n}\cdot i= (-1)^n \cdot i
            &\Rightarrow n \textrm{ is even}
                \Rightarrow z_k=0+1i \\
            &\Rightarrow n \textrm{ is odd}
                \Rightarrow z_k=0-1i \\
\end{align*}
\end{document}

但是当我尝试使用此代码在单一环境中执行此操作时：

\documentclass[fleqn, 12pt]{article}
\usepackage[utf8]{inputenc}
\usepackage{amsmath, amsthm, amssymb, mathtools}
\usepackage[margin=1.25in]{geometry}
\setlength{\mathindent}{0pt}
\setlength{\jot}{\baselineskip}
\usepackage[nodisplayskipstretch]{setspace} 
\setstretch{1.25} 

\begin{document}
\begin{align*}
    \Rightarrow k \textrm{ is even } (k=2n)
        \Rightarrow z_k=i^{2n}=(-1)^n 
            &\Rightarrow n \textrm{ is even}
                \Rightarrow z_k=1+0i \\
            &\Rightarrow n \textrm{ is odd}
                \Rightarrow z_k=-1+0i \\
    \Rightarrow k \textrm{ is odd } (k=2n+1)
        \Rightarrow z_k=i^{2n+1}=i^{2n}\cdot i= (-1)^n \cdot i
            &\Rightarrow n \textrm{ is even}
                \Rightarrow z_k=0+1i \\
            &\Rightarrow n \textrm{ is odd}
                \Rightarrow z_k=0-1i \\
\end{align*}
\end{document}

这是我得到的：

我理解这是因为 Latex 会将所有直箭头对齐（因为有“&”），同时使最长行的开头从右侧开始。所以我尝试在某些行的开头添加“&”，以使其看起来像我想要的那样，但没有成功。

提前感谢您的回答。

Answer 1

欢迎来到 TeX.SE！

您可以aligned在以下位置嵌套环境align*：

\documentclass[fleqn]{article}
\usepackage{amsthm, amssymb, mathtools}
\usepackage[margin=1.25in]{geometry}
\setlength{\mathindent}{0pt}
\setlength{\jot}{\baselineskip}

\begin{document}
\begin{align*}
&   \begin{aligned}
\Rightarrow k \textrm{ is even } (k=2n)
    \Rightarrow z_k=i^{2n}=(-1)^n
        &   \Rightarrow n \textrm{ is even}
                \Rightarrow z_k=1+0i \\
        &   \Rightarrow n \textrm{ is odd}
                \Rightarrow z_k=-1+0i 
    \end{aligned}   \\
%
&   \begin{aligned}
\Rightarrow k \textrm{ is odd } (k=2n+1)
    \Rightarrow z_k=i^{2n+1}=i^{2n}\cdot i= (-1)^n \cdot i
        &   \Rightarrow n \textrm{ is even}
                \Rightarrow z_k=0+1i \\
        &   \Rightarrow n \textrm{ is odd}
                \Rightarrow z_k=0-1i \\
    \end{aligned}
\end{align*}
\end{document}

Answer 2

我建议您alignedat对“内部”表达式使用环境，以便您\Rightarrow也可以对齐“内部”符号。

\documentclass[fleqn, 12pt]{article}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{amsmath, amsthm, amssymb, mathtools}
\usepackage[margin=1.25in]{geometry}
\setlength{\mathindent}{0pt}
\setlength{\jot}{\baselineskip}
\usepackage[nodisplayskipstretch]{setspace} 
\setstretch{1.25} 

\begin{document}
\begin{align*}
&\Rightarrow \textup{$k$ is even } (k=2n) \Rightarrow z_k=i^{2n}=(-1)^n 
   \begin{alignedat}[t]{2}
       &\Rightarrow \textup{$n$ is even} &{}\Rightarrow{}& z_k=\phantom{-}1+0i \\
       &\Rightarrow \textup{$n$ is odd}  &{}\Rightarrow{}& z_k=-1+0i 
   \end{alignedat}\\[0.5\jot]
&\Rightarrow \textup{$k$ is odd } (k=2n+1) \Rightarrow 
           z_k=i^{2n+1}=i^{2n}\cdot i= (-1)^n \cdot i
   \begin{alignedat}[t]{2}
       &\Rightarrow \textup{$n$ is even} &{}\Rightarrow{}& z_k=0+1i \\
       &\Rightarrow \textup{$n$ is odd}  &{}\Rightarrow{}& z_k=0-1i 
   \end{alignedat}
\end{align*}
\end{document}