How to center align equations and left align implication arrows

I'm trying to align multiple equations within a proof, and use implication arrows to denote next steps.

I want the equations to be centered like how normal align would, but I want the implication arrows to be left aligned as well.

I imagine the arrows looking like \tag*{$\implies$} but on the left-hand side.

I've tried different manipulations of the align environment, but can't achieve what I'm looking for.

\begin{align*}
a &= b \\
\implies 2a &= 2b \\
\implies f(2a) &= f(2b) \\
\end{align*}

\begin{align*}
&& a &= b \\
\implies&& 2a &= 2b \\
\implies&& f(2a) &= f(2b) \\
\end{align*}

\begin{align*}
& a &&= b \\
\implies& 2a &&= 2b \\
\implies& f(2a) &&= f(2b) \\
\end{align*}


• Welcome to TeX.SE. Please always provide a full MWE starting with \documentclass{}. Commented Sep 25, 2021 at 12:30

Like this?

(red liens indicate text area borders)

\documentclass{article}
\usepackage{amsmath}

\begin{document}
\begin{alignat*}{3}
&               &\quad       a & = b \\
&   \implies    &\quad      2a & = 2b \\
&   \implies    &\quad   f(2a) & = f(2b)
\end{alignat*}
\end{document}

\documentclass{article}
\usepackage{amsmath}

\begin{document}
\begin{flalign*}
&               &       a & = b     &   \\
&   \implies    &      2a & = 2b    &   \\
&   \implies    &   f(2a) & = f(2b) &
\end{flalign*}
\end{document}


Edit: or you looking for the following:

\documentclass{article}
\usepackage{amsmath}

\begin{document}
\begin{flalign*}
&               &       a & = b     &   \\
&   \implies    &      2a & = 2b    &   \\
&   \implies    &   f(2a) & = f(2b) &
\end{flalign*}
\end{document}

• This is the closest to what I was thinking! Is it possible to get the arrows fully left justified? Commented Sep 25, 2021 at 15:50
• @goatboy3million, you mean at left text area border? Commented Sep 25, 2021 at 16:34
• Yes, the edit was exactly what I was looking for! Thank you very much. Commented Sep 25, 2021 at 17:44

How about the ArrowBetweenLinescommand from mathtools?

\documentclass{article}
\usepackage{mathtools}

\begin{document}

\begin{alignat*}{2}
& & a &= b \\
\ArrowBetweenLines [\implies] & & 2a &= 2b \\
\ArrowBetweenLines [\implies] & & f(2a) &= f(2b) \\
\end{alignat*}

\end{document}


• cool idea, didn't know you could do that! though i think it would look cleaner if the arrows were on the same line as the statements Commented Sep 25, 2021 at 17:48
• It is designed to be between rows and by default it is an \Updownarrow, but in your case you also might give a try to \Downarrow for the optional argument. Commented Sep 25, 2021 at 19:15

You can try to do it manualy (but it is not very good example), and also is it what you need or I sisunderstand you?

\documentclass[a4paper,14pt]{extreport}
\usepackage{amssymb}
\usepackage{amsmath}
\begin{document}

\begin{align*}
a &= b \\
\implies  \quad\hspace{0.7cm}  2a &= 2b \\
\implies \quad f(2a) &= f(2b) \\
\end{align*}

\end{document}

• Better to use \hphantom {f()} instead of guessing a value. Commented Sep 25, 2021 at 13:29