# Weird spacing in aligned environment

Here's my code:

\begin{aligned} h_t &= ca_t + &2c^2a_{t+1} + 3c^3&a_{t+2}+\ldots \\ h_{t+1} &= &ca_{t+1} + 2c^2&a_{t+2} + 3c^3a_{t+3}+\ldots \\ h_{t+2} &= & c&a_{t+2} + 2c^2a_{t+3} + 3c^3a_{t+4}+\ldots \end{aligned}


and the picture:

1. What's with the weird space between ca_t + and 2c^2a_{t+1}? How can I get rid of it?
2. Although it looks correct, I don't understand why the picture has the a_{t+1} terms aligned when in the code I write that the 2c^2 and c should be aligned. Again, it looks right (and I should fix my code), but I'm just confused about why the picture looks the way it does.

I'd use an array environment, which is highly customizable.

\documentclass{article}
\usepackage{array} % for "\newcolumntype" directive
\newcolumntype{C}{>{{}}c<{{}}}
\newcommand\mydots{\multicolumn{1}{l}{\cdots}}

\begin{document}
$\setlength\arraycolsep{0pt} \begin{array}{l*{6}{Cr}} h_t &=&ca_t&+&2c^2a_{t+1}&+&3c^3a_{t+2}&+&\mydots \\ h_{t+1} &=& & &ca_{t+1} &+&2c^2a_{t+2}&+&3c^3a_{t+3}&+&\mydots \\ h_{t+2} &=& & & & & ca_{t+2}&+&2c^2a_{t+3}&+&3c^3a_{t+4}&+&\mydots \end{array}$
or, if lining up the $c$'' coefficients matters too,
$\setlength\arraycolsep{0pt} \begin{array}{l*{6}{Cr}} h_t &=&c\,a_t&+&2c^2a_{t+1}&+&3c^3a_{t+2}&+&\mydots \\ h_{t+1} &=& & &c^{\phantom{1}}a_{t+1} &+&2c^2a_{t+2}&+&3c^3a_{t+3}&+&\mydots \\ h_{t+2} &=& & & & & c^{\phantom{1}}a_{t+2}&+&2c^2a_{t+3}&+&3c^3a_{t+4}&+&\mydots \end{array}$
\end{document}


Is this what you want?

You have to remember that n alignment points require 2 n – 1 &. Also, using alignat* gives you full control on the spacing of the alignment columns.

Unrelated: don't use the TeX construct $$...$$, use the LaTeX construct $...$ (unnecessary here – use the starred version of the amsmath environments):

\documentclass[11pt]{article}
\usepackage{amsmath}

\begin{document}

\begin{alignat*}{3}
h_t &= ca_t &{}+2c^2 & a_{t+1} & {}+ 3c^3&a_{t+2}+\dotsb \\
h_{t+1} &= & c \,& a_{t+1} &{} + 2c^2&a_{t+2} + 3c^3a_{t+3}+\dotsb \\
h_{t+2} &= & & & c&a_{t+2} + 2c^2a_{t+3} + 3c^3a_{t+4}+\dotsb
\end{alignat*}

\end{document}


A variant of Mico's answer, with more alignment points, but with easier input, taking advantage of the repetitive structure.

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

\begin{document}
$\setlength{\arraycolsep}{0pt} \newcommand{\row}[1]{% & c^{\hphantom{1}} & a_{t\ifnum#1=0 \else+#1\fi} & + & 2 & c^2 & a_{t+\number\numexpr#1+1} & + & 3 & c^3 & a_{t+\number\numexpr#1+2} & + & \multicolumn{3}{l}{\dotsb} } \begin{array}{ l % the left hand sides >{{}}c<{{}} % the equals sign *{5}{ l % numeric coefficient l % power of c c % 'a' term >{{}}c<{{}} % plus } lll % final dots } h_t &=& \row{0} \\[\jot] h_{t+1} &=& &&&& \row{1} \\[\jot] h_{t+2} &=& &&&&&&&& \row{2} \end{array}$
\end{document}