How to make things symmetric around an arrow

Question

Can someone tell me what commando is needed to make expressions like in the text below. I mean like it is symmetric around the arrow.

Answer 1

Apart from the wrong dots, it seems an align*:

\documentclass{article}
\usepackage[margin=1in]{geometry}
\usepackage{amsmath,amssymb}
\usepackage{lipsum}

\begin{document}

\lipsum*[2]
\begin{align*}
\{B_1,\dots,B\cup\{\lnot\lnot\phi\},\dots,B_i\}\quad
  & \rightsquigarrow \quad
    \{B_1,\dots,B\cup\{\phi\},\dots,B_i\}
\\
\{B_1,\dots,B\cup\{\phi\land\psi\},\dots,B_i\}\quad
  & \rightsquigarrow \quad
    \{B_1,\dots,B\cup\{\phi,\psi\},\dots,B_i\}
\\
\{B_1,\dots,B\cup\{\lnot(\phi\land\psi)\},\dots,B_i\}\quad
  & \rightsquigarrow \quad
    \{B_1,\dots,B\cup\{\lnot\phi\},B\cup\{\lnot\psi\},\dots,B_i\}
\end{align*}
\lipsum[3]

\end{document}

Note that lipsum is just to produce dummy text and is not needed in your document.

Answer 2

You can use a regular array:

\documentclass{article}
\usepackage[margin=1in]{geometry}% Just for this example
\usepackage[nopar]{lipsum}% Just for this example
\usepackage{amsmath,amssymb}

\begin{document}

\lipsum[1]
\[
  \begin{array}{r@{\quad\rightsquigarrow\quad}l}
    \{ B_1, \dots, B \cup \{ \neg\neg\phi \}, \dots, B_i \} &
      \{ B_1, \dots, B \cup \{ \phi \}, \dots, B_i \} \\
    \{B_1, \dots, B \cup \{ \phi \wedge \psi \}, \dots, B_i\} &
      \{B_1, \dots, B \cup \{ \phi, \psi \}, \dots, B_i\} \\
    \{B_1, \dots, B \cup \{ \neg(\phi \wedge \psi) \}, \dots, B_i\} &
      \{B_1, \dots, B \cup \{ \neg\phi \}, B \cup \{ \neg\psi \}, \dots, B_i\}
  \end{array}
\]
\lipsum[2]

\end{document}

The column specification has a forced column separation causing the centred \rightsquigarrow.

Other options also exist with align-like environments from amsmath, if needed.