How to vertically align the following equations?

Question

Consider the following screenshot. I want to vertically align the left = as shown in the expected part.

\documentclass[border=12pt,preview]{standalone}
\usepackage{amsmath}
\begin{document}
\abovedisplayskip=0pt\relax
\begin{align*}
    a_1=4   &= 4 + 3 \times 0 \\
    a_2=7   &= 4 + 3 \times 1 \\
    a_3=10  &= 4 + 3 \times 2 \\
    a_4=13  &= 4 + 3 \times 3 \\
    a_5=16  &= 4 + 3 \times 4 \\
    a_6=19  &= 4 + 3 \times 5 \\
    a_7=22  &= 4 + 3 \times 6 
\end{align*}
\end{document}

Answer 1

You can use either alignat or array.

The first solution has already been proposed; I use {} after every &= for uniformity, even if just one would suffice.

Using array spares this and the result is the same. On the other hand, array would be much easier if you also need other alignment points, say to go up to a₁₁

\documentclass{article}
\usepackage{amsmath}
\usepackage{array}

\begin{document}

\begin{alignat*}{2}
% R    L      R   L
  a_1 &={} &  4  &= 4 + 3 \times 0 \\
  a_2 &={} &  7  &= 4 + 3 \times 1 \\
  a_3 &={} & 10  &= 4 + 3 \times 2 \\
  a_4 &={} & 13  &= 4 + 3 \times 3 \\
  a_5 &={} & 16  &= 4 + 3 \times 4 \\
  a_6 &={} & 19  &= 4 + 3 \times 5 \\
  a_7 &={} & 22  &= 4 + 3 \times 6 
\end{alignat*}

\begin{equation*}
\renewcommand{\arraystretch}{1.2}
\setlength{\arraycolsep}{0pt}
\begin{array}{@{} l >{{}}c<{{}} r >{{}}l @{}}
  a_1 &= &  4  &= 4 + 3 \times 0 \\
  a_2 &= &  7  &= 4 + 3 \times 1 \\
  a_3 &= & 10  &= 4 + 3 \times 2 \\
  a_4 &= & 13  &= 4 + 3 \times 3 \\
  a_5 &= & 16  &= 4 + 3 \times 4 \\
  a_6 &= & 19  &= 4 + 3 \times 5 \\
  a_7 &= & 22  &= 4 + 3 \times 6 
\end{array}
\end{equation*}

\end{document}

\documentclass{article}
\usepackage{amsmath}
\usepackage{array}

\begin{document}

\begin{equation*}
\renewcommand{\arraystretch}{1.2}
\setlength{\arraycolsep}{0pt}
\begin{array}{@{} l >{{}}c<{{}} r >{{}}l @{}}
  a_1    &= &  4  &= 4 + 3 \times 0 \\
  a_2    &= &  7  &= 4 + 3 \times 1 \\
  a_3    &= & 10  &= 4 + 3 \times 2 \\
  a_4    &= & 13  &= 4 + 3 \times 3 \\
  a_5    &= & 16  &= 4 + 3 \times 4 \\
  a_6    &= & 19  &= 4 + 3 \times 5 \\
  a_7    &= & 22  &= 4 + 3 \times 6 \\
  a_{11} &= & 34  &= 4 + 3 \times 10
\end{array}
\end{equation*}

\end{document}

Answer 2

This will work:

\documentclass[border=12pt,preview]{standalone}
\usepackage{amsmath}
\begin{document}
\abovedisplayskip=0pt\relax
\begin{align*}
    a_1&=\phantom{0}4   = 4 + 3 \times 0 \\
    a_2&=\phantom{0}7   = 4 + 3 \times 1 \\
    a_3&=10  = 4 + 3 \times 2 \\
    a_4&=13  = 4 + 3 \times 3 \\
    a_5&=16  = 4 + 3 \times 4 \\
    a_6&=19  = 4 + 3 \times 5 \\
    a_7&=22  = 4 + 3 \times 6 
\end{align*}
\end{document}

Answer 3

With alignat:

\documentclass[border=12pt,preview]{standalone}
\usepackage{amsmath}

\begin{document}

\abovedisplayskip=0pt\relax
\begin{alignat*}{2}
    a_1 & =&4 &= 4 + 3 \times 0 \\
    a_2 & = & 7&= 4 + 3 \times 1 \\
    a_3 & ={}&10 &= 4 + 3 \times 2 \\
    a_4 & = &13 &= 4 + 3 \times 3 \\
    a_5 & = &16 &= 4 + 3 \times 4 \\
    a_6 & = &19 &= 4 + 3 \times 5 \\
    a_7 & = &22 &= 4 + 3 \times 6
\end{alignat*}

\end{document}