略圆的尖括号

Question

如何才能得到稍微圆角的尖括号？以下是所需结果的手写示例。

Answer 1

经过一番研究，我发现使用 tikz 可以获得足够接近原始想法的结果。

\documentclass{article}
\usepackage{xparse}
\usepackage{amsmath}
\usepackage{tikz}

\newcommand{\lroungle}{\mathrel{\mkern-4mu
        \raisebox{-2.1pt}{%
            \tikz[line cap=round, line join=round]
            \draw
            (0ex, 2.2ex) to[out=-135,in=135, distance=7]     (0ex,0ex)
            ;%
        }\mkern-3mu}
}
\newcommand{\rroungle}{%
    \mathrel{\mkern-4mu\raisebox{-2.1pt}{%
            \tikz[line cap=round, line join=round]
            \draw
            (0ex, 2.2ex) to[out=-45,in=45, distance=7]   (0ex,0ex)
            ;%
    }}
}
\begin{document}
    \[\lroungle A , B \rroungle \]
\end{document}

此外，我注意到可以稍微修改物理包中的 brackett 命令，用圆角括号代替标准角括号。

\documentclass{article}
\usepackage{xparse}
\usepackage{amsmath, amsfonts}
\usepackage{tikz, scalerel}


\newcommand{\lroungle}{\mathrel{\mkern-4mu
        \raisebox{-2.1pt}{%
            \tikz[line cap=round, line join=round]
            \draw
            (0ex, 2.2ex) to[out=-135,in=135, distance=7]     (0ex,0ex)
            ;%
        }\mkern-3mu}
}
\newcommand{\rroungle}{%
    \mathrel{\mkern-4mu\raisebox{-2.1pt}{%
            \tikz[line cap=round, line join=round]
            \draw
            (0ex, 2.2ex) to[out=-45,in=45, distance=7]   (0ex,0ex)
            ;%
    }}
}

\DeclareDocumentCommand\rbra{ s m t\rket s g }
{ % Bra
\IfBooleanTF{#3}
{ % Contraction
    \IfBooleanTF{#1}
    { % Bra has a star: no resize
        \IfNoValueTF{#5}
        {\rbraket*{#2}{} \IfBooleanTF{#4}{*}{}}
        {\rbraket*{#2}{#5}}
    }
    {
        \IfBooleanTF{#4}
        { % Ket has a star: no resize
            \IfNoValueTF{#5}
            {\rbraket{#2}{} *}
            {\rbraket*{#2}{#5}}
        }
        {\rbraket{#2}{\IfNoValueTF{#5}{}{#5}}} % Neither term is starred: auto sizing
    }
}
{ % No contraction
    \IfBooleanTF{#1}
    {\lroungle \smash{#2} \rvert}
    {\stretchleftright{\lroungle\mkern2.5mu}{#2}{\rvert}}
    \IfBooleanTF{#4}{*}{}
    \IfNoValueTF{#5}{}{#5}
}
}

\DeclareDocumentCommand\rket{ s m }
{ % Ket
\IfBooleanTF{#1}
{\vphantom{#2}\lvert\smash{#2}\rroungle} % No resize
{\stretchleftright{\lvert}{#2}{\mkern2.5mu\rroungle}} % Auto sizing
}

\DeclareDocumentCommand\rinnerproduct{ s m g }
{ % Inner product
\IfBooleanTF{#1}
{ % No resize
    \IfNoValueTF{#3}
    {\vphantom{#2}\lroungle\smash{#2}\vert\smash{#2}\rroungle}
    {\vphantom{#2#3}\lroungle\smash{#2}\vert\smash{#3}\rroungle}
}
{ % Auto resize
    \IfNoValueTF{#3}
    {\stretchleftright{\lroungle}{#2}{\vert} #2 \stretchrel*{\rroungle} 
{\vphantom{#2}}}
    {\stretchleftright{\lroungle}{#2 \vphantom{#3}}{\vert} {#3}\stretchrel* 
{\rroungle}{\vphantom{#2#3}}}
}
}
\DeclareDocumentCommand\rbraket{}{\rinnerproduct} 

\begin{document}
\newcommand{\kkt}{\widetilde{\mathbf k}}
\newcommand{\eps}{\varepsilon}
\begin{equation*}
\sum_{\mathbf k \in 2\pi \mathbf{Z}^3} \rket*{\mathbf k}\rbra*{\mathbf k}=\mkern-8mu \sum_{\kkt \in 2\pi\eps \mathbf{Z}^3}\rket{\frac{\kkt}{\eps}} \rbra{\frac{\kkt}{\eps}}
\end{equation*} 
\end{document}